Compositions and methods for reducing h2s levels in fermented beverages

ABSTRACT

The present invention provides compositions and methods for reducing H 2 S levels in fermented beverages.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 60/918,616, filed Mar. 16, 2007, and U.S. Provisional Application No. 60/959,366, filed Jul. 12, 2007, the entire disclosure of both of which are hereby incorporated by reference for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The production of volatile sulfur compounds such as hydrogen sulfide (H₂S) during alcoholic fermentation is an issue that affects the brewing and winemaking industries. Hydrogen sulfide (H₂S) is an undesirable by-product of the sulfate reduction pathway (FIG. 1). It is formed in Saccharomyces cerevisiae under fermentation conditions. Production of H₂S by S. cerevisiae strains ranges from 0 ug/L to 290 ug/L, well above the human detection threshold of 11 ng/L (Amoore and Hautala 1983). Its undesirable quality stems from the fact that it introduces a rotten egg odor characteristic to wines and although H₂S is a volatile compound and can be removed by aeration, it has the potential to form mercaptans and thiols which will persist in the wine due to the low pH (Thoukis 1962). Mercaptans and thiols present themselves as onion or canned vegetable aromas and where volatile H₂S can be managed, removal of other undesired sulfur compounds is technically difficult and strips the wine of other flavor compounds.

The formation of hydrogen sulfide by Saccharomyces cerevisae is a well-documented problem in the wine, beer and sake industry (Acree et al. 1972, Eschenbruch et al. 1978, Giudici and Kunkee 1994, Jiranek et al. 1995, Rauhut and Kurbel 1994, Walker and Simpson 1993). Nutritional factors such as levels of nitrogen, vitamins and cofactors (Giudici and Kunkee 1994, Jiranek et al. 1995) and environmental factors such as temperature, pH, levels of elemental sulfur (Rauhut and Kurbel 1994), presence of sulfur dioxide (Stratford and Rose 1985) and levels of organic compounds containing sulfur (Acree et al. 1972) have been associated with the production of volatile sulfur compounds in fermented beverages. The differences in production of volatile sulfur compounds have also been attributed to differences in yeast strain metabolism (Acree et al. 1972, Spiropoulos et al. 2000).

There are at least six different classes of yeast strain behavior with respect to hydrogen sulfide formation: 1) elevated levels under all conditions; 2) low levels under all conditions; 3) elevated production below and above a threshold level of nitrogen; reduced production during a ‘window’ of nitrogen levels with sulfide increasing at nitrogen levels above or below this window; 4) elevated production in response to limiting micronutrient levels irrespective of nitrogen content; 5) elevated sulfide production only when limited for both nitrogen and micronutrients; and 6) elevated sulfide production with increased rate of fermentation, which may be related to fermentation rate and carbon dioxide evolution or to some other factor such as increased heat production (Spiropoulos 2000, Jiranek 1995, Giudici 1994, Linderholm 2006).

The existing method for stripping sulfides from wine is copper fining. Copper addition can lead to the catalysis of deleterious compositional changes as well as increase the amount of waste produced by wineries requiring special treatment, ultimately resulting in higher production costs for wineries and higher wine costs for the consumer. Further, use of copper as a fining agent may lead to high residual copper levels in wine. The Trade and Tax Bureau allows a residual copper level of 0.5 mg/L for wine (See, e.g., the worldwide website at regulations.justia.com/view/89060/). Winemakers who use copper to remove hydrogen sulfide must then take measures to reduce the copper levels in the wine. Given the adverse health effects associated with excessive copper ingestion, particularly neurological disorders such as Alzheimers, the World Health Organization has recommended dietary restrictions on consumption of this compound (See, the worldwide website at who.int/water_sanitation_health/dwq/chemicals/copper.pdf). The availability of commercial yeast strains unable to produce hydrogen sulfide or which produce reduced levels of hydrogen sulfide will eliminate the need for copper treatment of wines.

Thus, there is a need in the art for compositions and methods for reducing H₂S levels in fermented beverages. The present invention meets these and other needs.

BRIEF SUMMARY OF THE INVENTION

The present invention provides compositions and methods for reducing H₂S levels in fermented beverages.

One aspect of the invention provides methods for reducing or eliminating H₂S levels in fermentation product or medium. In some embodiments, the methods comprise contacting the fermentation product or medium with a yeast strain, yeast cell or yeast culture comprising a polynucleotide encoding a modified MET10 polypeptide that does not catalyze the release of free hydrogen sulfide from sulfite (i.e., a “sulfide inactive” MET10 polypeptide), wherein the amino acid at position 662 of the MET10 polypeptide is not threonine. In some embodiments, the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is not threonine. In some embodiments, the polynucleotide comprises SEQ ID NO:1.

With respect to the embodiments of a sulfide inactive MET10 polypeptide, in some embodiments, the amino acid residue at position 662 of the MET10 polypeptide is not threonine or serine. In some embodiments, the amino acid residue at position 662 of the MET10 polypeptide is Ala, Cys, Asp, Glu, Phe, Gly, His, Ile, Lys, Leu, Met, Asn, Pro, Gln, Arg, Ser, Val, Trp or Tyr (SEQ ID NO:3). In some embodiments, the amino acid residue at position 662 of the MET10 polypeptide is Ala, Cys, Asp, Glu, Phe, Gly, His, Ile, Lys, Leu, Met, Asn, Pro, Gln, Arg, Val, Trp or Tyr (SEQ ID NO:5). In some embodiments, the amino acid residue at position 662 is selected from the group consisting of Lys, Arg, His, Gln and Asn (SEQ ID NO:6). In some embodiments, the amino acid residue at position 662 is Lys (SEQ ID NO:7).

In some embodiments, the sulfide inactive MET10 polypeptide or MET10 polynucleotide is a yeast MET10. In some embodiments, the sulfide inactive MET10 polypeptide shares at least about 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or higher sequence identity with a MET10 of SEQ ID NO:3 or SEQ ID NO:4, wherein X at position 662 is as described above and herein. In some embodiments, the polynucleotide encoding a sulfide inactive MET10 polypeptide shares at least about 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or higher sequence identity with SEQ ID NO:1.

In some embodiments, the yeast cell does not also express a sulfide active MET10 polypeptide capable of converting sulfite into sulfide. In some embodiments, the fermentation product is wine, beer or champagne. In some embodiments, the fermentation media may be selected from the group consisting of a must (e.g., a grape juice must) and a wort.

With respect to the embodiments of yeast cells, in some embodiments, the yeast strain may be a Saccharomyces cerevisiae strain. In some embodiments, the yeast strain can be any commercially available strain for use with making beer or wine, as described herein. Oftentimes, the parent strain or originating strain is a hydrogen sulfide producer that has been rendered a hydrogen sulfide non-producer by replacement of the nucleic acid encoding a sulfide active MET10 polypeptide with a nucleic acid encoding a sulfide inactive MET10 polypeptide. Exemplary S. cerevisiae wine strains include, without limitation, Prise de Mousse, Premier Cuveé, French Red, Montachet, Lallemand K1, Bordeaux, UCD522, UCD940, Ba25, Ba126, Ba137, Ba220, Bb23, Bb25, Ba30, Bb32, Bb19 and Bb22. Further embodiments of yeast cells are as described herein.

Another aspect of the invention provides isolated polynucleotides comprising a nucleic acid sequence that encodes a MET10 polypeptide that does not catalyze the conversion of sulfite into sulfide, wherein the amino acid at position 662 of the MET10 polypeptide is not threonine. In some embodiments the amino acid at position 662 of the MET10 polypeptide is not threonine or serine (SEQ ID NO:5). The embodiments of the sulfide inactive MET10 polypeptide encoded by the polynucleotides are as described above and herein. In some embodiments, the isolated polynucleotide encoding a sulfide inactive MET10 polypeptide shares at least about 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% sequence identity with SEQ ID NO:1. In some embodiments, the isolated polynucleotide comprises the nucleic acid sequence provided in SEQ ID NO:1 or a complement thereof.

In a related aspect, the invention provides expression cassettes and expression vectors comprising a polynucleotide encoding a MET10 polypeptide that does not catalyze the conversion of sulfite into sulfide, wherein the amino acid at position 662 of the MET10 polypeptide is not threonine (SEQ ID NO:3), and wherein the polynucleotide is operably linked to an expression control sequence. Further embodiments of the sulfide inactive MET10 polypeptide are as described above and herein. Further provided are host cells comprising the expression vectors or expression cassettes. The host cells can be yeast cells, for example, Saccharomyces cerevisiae cells. Further embodiments of the yeast cells are as described above and herein. In some embodiments, the expression cassette or expression vector comprise a promoter that promotes expression in a yeast cell.

In a related aspect, the invention provides improved yeast cells that do not produce detectable hydrogen sulfide or produce low levels of hydrogen sulfide, the improved cells comprising an exogenous polynucleotide encoding a MET10 polypeptide that does not catalyze the conversion of sulfite to sulfide, wherein the amino acid at position 662 of the MET10 polypeptide is not threonine (SEQ ID NO:3), wherein a parent cell of the improved yeast cell produces hydrogen sulfide. In some embodiments, the amino acid at position 662 of the MET10 polypeptide is not threonine or serine (SEQ ID NO:5). Further embodiments of the sulfide inactive MET10 polypeptides and yeast cells are as described above and herein.

In a further aspect, the invention provides improved yeast cell cultures that produce reduced levels of or do not produce detectable hydrogen sulfide, the improved culture comprising a population of yeast cells, the yeast cells comprising an exogenous polynucleotide encoding a MET10 polypeptide that does not catalyze the conversion of sulfite to sulfide, wherein the amino acid at position 662 of the MET10 polypeptide is not threonine (SEQ ID NO:3), wherein the improved yeast cell culture produces no or reduced hydrogen sulfide in comparison to a culture of parent cells. In some embodiments, the amino acid at position 662 of the MET10 polypeptide is not threonine or serine (SEQ ID NO:5). Further embodiments of the sulfide inactive MET10 polypeptides and yeast cells are as described above and herein.

In another aspect, the invention provides methods of producing an improved yeast cell that does not produce detectable hydrogen sulfide, the method comprising replacing an endogenous nucleic acid encoding a sulfide active MET10 polypeptide with a nucleic acid encoding a sulfide inactive MET10 polypeptide by introducing into a parent of the improved yeast cell an exogenous polynucleotide encoding a sulfide inactive MET10 polypeptide that does not catalyze the conversion of sulfite to sulfide, wherein the amino acid at position 662 of the sulfide inactive MET10 polypeptide is not threonine (SEQ ID NO:3), wherein the parent of the improved yeast cell produces hydrogen sulfide. In some embodiments, the amino acid at position 662 of the MET10 polypeptide is not threonine or serine (SEQ ID NO:5). In some embodiments, the nucleic acid encoding the sulfide inactive MET10 polypeptide is introduced recombinantly. In some embodiments, the nucleic acid encoding the sulfide inactive MET10 polypeptide is introduced by back-crossing. Further embodiments of the sulfide inactive MET10 polypeptides and yeast cells are as described above and herein.

In another aspect, the invention provides fermentation products, e.g., wine, beer, champagne, with no detectable hydrogen sulfide or low levels of hydrogen sulfide, or residue therefrom, wherein the fermentation products are produced according to the methods described herein.

A further embodiment of the invention provides isolated polynucleotides capable of distinguishing between the sequences provided in SEQ ID NO:1 or a complement thereof and a nucleic acid encoding a wild type METIO, expression vectors comprising the polynucleotides operably linked to an expression control sequence, and host cells (e.g., Saccharomyces cerevisiae cells) comprising the expression vector.

A further embodiment of the invention provides isolated polynucleotides comprising one or more substitutions (e.g., at least two, three, four or more substitutions) in SEQ ID NO:1, wherein the one or more substitutions are selected from: an A→C at position 404, an A→G at position 514, an A→G at position 1278, and a C→T at position 1532, a G→A at position 1768, and an A→C at position 1985, expression vectors comprising the polynucleotides operably linked to an expression control sequence, and host cells (e.g., Saccharomyces cerevisiae cells) comprising the expression vector.

Yet another embodiment of the invention provides isolated polynucleotides comprising one or more substitutions (e.g., at least two, three, four or more substitutions) in SEQ ID NO:2, wherein the one or more substitutions are selected from the group consisting of: a C→A at position 404, a G→A at position 514, a G→A at position 1278, and a T→C at position 1532, an A→G at position 1768, and a C→A at position 1985, expression vectors comprising the polynucleotides operably linked to an expression control sequence, and host cells (e.g., Saccharomyces cerevisiae cells) comprising the expression vector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the sulfate reduction pathway. Sequence analysis conducted for genes is in bold, number of alleles found is in ( ) alleles found in UCD932 are outlined with dotted lines.

FIGS. 2A-2Z set forth a sequence alignment of the MET10 allele in various Saccharomyces strains (SEQ ID NOS:8, 9, 1, 10-14, 2, 15 and 16, respectively). Nucleic acid changes that result in codon changes are highlighted.

FIG. 3 illustrates an exemplary gene swapping technique. MET10^(S)=S288C allele. MET10^(W)=Wine strain allele.

FIG. 4 illustrates the amino acid sequence alignment of the MET10 gene in various Saccharomyces strains (S288C=SEQ ID NO:17; UCD932=SEQ ID NO:18; UCD950=SEQ ID NO:19). Amino acid differences between the different strains are highlighted.

FIG. 5 illustrates the amino acid changes surrounding residue 662 in the MET10 protein. DNA sequences of MET10 alleles from S288C (SEQ ID NO:20), UCD932 (SEQ ID NO:21) and UCD950 (SEQ ID NO:20) aligned near nucleotide 1985 with the key mutation highlighted and bolded. The codons (SEQ ID NOS:22 and 23) and corresponding amino acid sequence (SEQ ID NO:24) (highlighted in light gray) are shown in the inset. The change from a C to an A results in the corresponding change of a threonine residue to a lysine at position 662 of the protein.

FIG. 6 illustrates the location of the 662 amino acid residue with respect to the known and predicted functional domains of the MET10 protein. A map of the known motifs and domains of the MET10 protein is depicted. The position of the altered base at position 662 is marked by the black arrow. The mutation resides within the sulfite reductase domain of the protein. Data from the world wide web at //db.yeastgenome.org/cgi-bin/protein/domainPage.pl?dbid=S000001926.

FIG. 7 illustrates the structural features of the sulfite reductase domain and illustrates the impact of the change of the threonine residue for a lysine on the structural features of the protein. Structural ribbon models of the MET10 protein based on structural homology prediction are depicted. Only the predicted sulfite reductase domain from lysine 633 to tyrosine 1035 is shown with the region around residue 662 enlarged in the inset. The predicted structure for UCD932 (Figure A) highlights the lysine at residue 662 while the predicted structure for UCD950 (Figure B) highlights the Threonine at residue 662.

FIG. 8 illustrates an alignment of a subsequence of MET10 protein from some industrially relevant yeast species (UCD 932 Met10=SEQ ID NO:25; S288c Met10=SEQ ID NO:26; S. cerevisiae (carlsbergensis)=SEQ ID NO:27; Kluyveromyces lactis=SEQ ID NO:28; Yarowwia lipolytica=SEQ ID NO:29; Schizosaccharomyces pombe=SEQ ID NO:30) whose sequences in the sulfite reductase catalytic region are known. Amino acid residues conserved throughout the aligned species are in bold. Amino acid residues conserved in the most related species are shaded. The threonine at position 662 or within the motif (N/K)(R/K)R(V/L)TP(A/D/E)(D/N/E)Y(D/N)R(Y/N)IFH(I/V)EFD(I/L) (SEQ ID NO:31) is conserved in the active MET10 polypeptide throughout all yeast species aligned.

BRIEF DESCRIPTION OF THE TABLES

Table 1 sets forth a list of native and industrial yeast strains.

Table 2 sets forth composition for a modified Triple M (MMM) media.

Table 3 sets forth results from the analysis of native yeast isolates grown on BiGGY plates and MMM.

Table 4 sets forth additional yeast strains.

Table 5 sets forth sequences for PCR primers for amplifying, inter alfa, MET10.

Table 6 sets forth sets forth sequences for sequencing primers for inter alfa, MET10.

Table 7 sets forth results summarizing H₂S production from yeast strains transformed with MET10.

Table 8 sets forth amino acid differences in MET10 alleles.

Table 9 sets forth results summarizing H₂S production by additional yeast strains transformed with MET10.

Table 10 sets forth results summarizing H₂S production by yeast strains transformed with MET10.

Table 11 sets forth results summarizing H₂S production by yeast strains transformed with MET10 alleles.

DETAILED DESCRIPTION OF THE INVENTION Introduction

The present invention provides compositions and methods for reducing H₂S levels in fermented beverages. The invention is based in part on the discovery that a MET10 polypeptide with an amino acid residue at position 662 that is other than a threonine does not catalyze the conversion of sulfite to free or released hydrogen sulfide. This is exemplified by the expression of a sulfide inactive MET10 polypeptide from the MET10 allele in yeast strain UCD932 in which a single nucleotide change at position 1985 in the MET10 gene results in an amino acid change at position 662 from threonine to lysine in the catalytic domain of the MET10 protein.

II. Definitions

Unless defined otherwise, all technical and scientific terms used herein generally have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Generally, the nomenclature used herein and the laboratory procedures in cell culture, molecular genetics, organic chemistry and nucleic acid chemistry and hybridization described below are those well known and commonly employed in the art. Standard techniques are used for nucleic acid and peptide synthesis. Generally, enzymatic reactions and purification steps are performed according to the manufacturer's specifications. The techniques and procedures are generally performed according to conventional methods in the art and various general references (see generally, Sambrook and Russell, Molecular Cloning, A Laboratory Manual (3d ed., Cold Spring Harbor Laboratory Press 2001); Ausubel, et al., eds., Current Protocols in Molecular Biology (John Wiley & Sons 1987-2008)), which are provided throughout this document. The nomenclature used herein and the laboratory procedures in analytical chemistry, and organic synthesis described below are those well known and commonly employed in the art. Standard techniques, or modifications thereof, are used for chemical syntheses and chemical analyses.

“Fermentation media” or “fermentation medium” as used herein refers to an unfermented mixture prior to addition of yeast. Fermentation media include, e.g., musts and worts. Fermentation media may further comprise an additional sugar source (e.g., honey, cane sugar, beet sugar, corn sugar, fructose, sucrose, or glucose); acid (e.g., citric acid, malic acid, tartaric acid, and mixtures thereof) and yeast nutrients (e.g., diammonium phosphate or another nitrogen source, vitamins, and the like).

A “must” as used herein refers to an unfermented mixture of fruit juice, stem fragments, fruit skins, seeds and/or pulp produced by mashing the fruit. Any fruits containing fermentable sugar such as, for example, grapes, apples, cherries, peaches, nectarines, plums, apricots, pears, persimmons, pineapples, mangoes, kiwis, strawberries, raspberries, blueberries, elderberries, blackberries, cranberries, figs, and loquats can be used. The fruits may be dried, boiled, poached, or otherwise processed prior to mashing. A must may comprise two or more fruits.

“Wort” as used herein refers to an unfermented liquid produced by mashing grains and/or grain hulls. Any grains containing fermentable sugar such as, for example, barley, wheat, rye, barley, rice, corn and oats can be used. The grains may be roasted, flaked, or otherwise processed prior to mashing. A wort may be produced from a mixture comprising two or more grains.

“Met10” and “MET10” as used herein refers to the a subunit of assimilatory sulfite reductase of Saccharomyces. Functionally, a MET10 polypeptide catalyzes the conversion of sulfite into sulfide. Structurally, MET10 polypeptides, particularly yeast MET10 polypeptides, have been characterized. MET10 polypeptides contain a conserved sulfite reductase catalytic domain at the C-terminal portion, as well as FAD and NAD binding domains. The center portion of the polypeptide contains a pyruvate-ferredoxin oxidoreductase domain. In sulfide active MET10 polypeptides, that are capable of catalyzing the conversion of sulfite to free or released hydrogen sulfide, the amino acid residue at position 662 has been conserved, and is usually a threonine or sometimes a serine, particularly in yeast. Identified MET10 polypeptide domains are depicted in FIG. 6.

As used herein, “MET10” refers to nucleic acids and polypeptide polymorphic variants, alleles, mutants, and interspecies homologs that: (1) have an amino acid sequence that has greater than about 80% amino acid sequence identity, for example, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% or greater amino acid sequence identity, preferably over a region of at least about 25, 50, 100, 200, 500, 1000, or more amino acids, or over its full length, to a reference amino acid sequence encoded by a MET10 nucleic acid (for a yeast MET10 nucleic acid sequence, see, e.g. SEQ ID NO: 1, FIG. 2, and the exemplified GenBank accession numbers below); (2) bind to antibodies, e.g., polyclonal antibodies, raised against an immunogen comprising an amino acid sequence of a MET10 polypeptide (e.g., encoded by SEQ ID NO: 1), and conservatively modified variants thereof; (3) specifically hybridize under stringent hybridization conditions to an anti-sense strand corresponding to a nucleic acid sequence encoding a MET10 protein, and conservatively modified variants thereof; (4) have a nucleic acid sequence that has greater than about 95%, preferably greater than about 96%, 97%, 98%, 99%, or higher nucleotide sequence identity, preferably over a region of at least about 25, 50, 100, 200, 500, 1000, or more nucleotides, or over its full length, to a MET10 reference nucleic acid sequence. The nucleic acids and proteins of the invention include both naturally occurring or recombinant molecules. In some embodiments, the MET10 polypeptides and MET10 polynucleotides are from yeast. Exemplified MET 10 amino acid and nucleic acid sequences are set forth in Genbank Accession Nos. EF058164, EF058165, EF058166, EF058167, EF058168, EF058169, EF058170, EF058171, EF058172, EF058173.

As used herein, a “sulfide active MET10” polypeptide is capable of catalyzing the conversion of sulfite to hydrogen sulfide. In yeast, a sulfide active MET10 polypeptide may have a serine or threonine residue at amino acid position 662. In yeast strains, the amino acid at position 662 in S. cerevisiae is conserved as a threonine or a serine and resides in the following motif in the sulfite reductase catalytic region: (N/K)(R/K)R(V/L)TP(A/D/E)(D/N/E)Y(D/N)R(Y/N)IFH(I/V)EFD(I/L) (SEQ ID NO:31). See, FIG. 8.

As used herein, a “sulfide inactive MET10” polypeptide does not catalyze the conversion of sulfite to free or released hydrogen sulfide. In yeast, a sulfide inactive MET10 polypeptide will not have a threonine at amino acid position 662 or within the motif (N/K)(R/K)R(V/L)XP(A/D/E)(D/N/E)Y(D/N)R(Y/N)IFH(I/V)EFD(I/L) (SEQ ID NO:32), i.e., wherein X is not T. In some embodiments, a sulfide inactive MET10 polypeptide will not have a threonine or a serine residue at amino acid position 662. In some embodiments, the sulfide inactive MET10 polypeptide will have a Ala, Cys, Asp, Glu, Phe, Gly, His, Ile, Lys, Leu, Met, Asn, Pro, Gln, Arg, Ser, Val, Trp or Tyr at position 662 (SEQ ID NO:3). In some embodiments, the amino acid residue at position 662 in a sulfide inactive MET10 polypeptide does not have a hydroxyl group, for example, is not Thr, Ser, or Tyr (SEQ ID NO:33). In some embodiments, the amino acid residue at position 662 in a sulfide inactive MET10 polypeptide is a large or bulky amino acid, for example, Lys, Arg, His, Gln, Asn, Glu, Asp, Ile, Leu, Val, Phe, Tyr, or Trp (SEQ ID NO:34). In some embodiments, the amino acid residue at position 662 in a sulfide inactive MET10 polypeptide is a basic or positively charged amino acid, for example, Lys, Arg, His, Gln or Asn (SEQ ID NO:6). In some embodiments, the amino acid residue at position 662 is Lys (SEQ ID NO:7).

As used herein, an “exogenous” MET10 nucleic acid sequence or amino acid sequence is introduced into a parent yeast cell or parent yeast strain by the action of man. The introduction into the yeast cell of the exogenous nucleic acid sequence or exogenous amino acid sequence can be by any means known in the art, including recombinant methods or classical yeast breeding methods (e.g., back-crossing).

The terms “nucleic acid” and “polynucleotide” are used interchangeably herein to refer to deoxyribonucleotides or ribonucleotides and polymers thereof in either single- or double-stranded form. The term encompasses nucleic acids containing known nucleotide analogs or modified backbone residues or linkages, which are synthetic, naturally occurring, and non-naturally occurring, which have similar binding properties as the reference nucleic acid, and which are metabolized in a manner similar to the reference nucleotides. Examples of such analogs include, without limitation, phosphorothioates, phosphoramidates, methyl phosphonates, chiral-methyl phosphonates, 2-O-methyl ribonucleotides, peptide-nucleic acids (PNAs).

Unless otherwise indicated, a particular nucleic acid sequence also encompasses conservatively modified variants thereof (e.g., degenerate codon substitutions) and complementary sequences, as well as the sequence explicitly indicated. Specifically, degenerate codon substitutions may be achieved by generating sequences in which the third position of one or more selected (or all) codons is substituted with mixed-base and/or deoxyinosine residues (Batzer et al., Nucleic Acid Res. 19:5081 (1991); Ohtsuka et al., J. Biol. Chem. 260:2605-2608 (1985); Rossolini et al., Mol. Cell. Probes 8:91-98 (1994)). The term nucleic acid is used interchangeably with gene, cDNA, mRNA, oligonucleotide, and polynucleotide.

A nucleic acid “capable of distinguishing” as used herein refers to a polynucleotide(s) that (1) specifically hybridizes under stringent hybridization conditions to an anti-sense strand corresponding to a nucleic acid sequence encoding a MET10 protein, and conservatively modified variants thereof; or (2) has a nucleic acid sequence that has greater than about 80%, 85%, 90%, 95%, preferably greater than about 96%, 97%, 98%, 99%, or higher nucleotide sequence identity, preferably over a region of at least about 25, 50, 100, 200, 500, 1000, or more nucleotides, to a MET10 nucleic acid.

The phrase “stringent hybridization conditions” refers to conditions under which a probe will hybridize to its target subsequence, typically in a complex mixture of nucleic acid, but to no other sequences. Stringent conditions are sequence-dependent and will be different in different circumstances. Longer sequences hybridize specifically at higher temperatures. An extensive guide to the hybridization of nucleic acids is found in Tijssen, Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Probes, “Overview of principles of hybridization and the strategy of nucleic acid assays” (1993). Generally, stringent conditions are selected to be about 5-10° C. lower than the thermal melting point I for the specific sequence at a defined ionic strength Ph. The T_(m) is the temperature (under defined ionic strength, Ph, and nucleic concentration) at which 50% of the probes complementary to the target hybridize to the target sequence at equilibrium (as the target sequences are present in excess, at T_(m), 50% of the probes are occupied at equilibrium). Stringent conditions will be those in which the salt concentration is less than about 1.0 M sodium ion, typically about 0.01 to 1.0 M sodium ion concentration (or other salts) at Ph 7.0 to 8.3 and the temperature is at least about 30° C. for short probes (e.g., 10 to 50 nucleotides) and at least about 60° C. for long probes (e.g., greater than 50 nucleotides). Stringent conditions may also be achieved with the addition of destabilizing agents such as formamide. For selective or specific hybridization, a positive signal is at least two times background, optionally 10 times background hybridization. Exemplary stringent hybridization conditions can be as following: 50% formamide, 5×SSC, and 1% SDS, incubating at 42° C., or, 5×SSC, 1% SDS, incubating at 65° C., with wash in 0.2×SSC, and 0.1% SDS at 65° C.

Nucleic acids that do not hybridize to each other under stringent conditions are still substantially identical if the polypeptides which they encode are substantially identical. This occurs, for example, when a copy of a nucleic acid is created using the maximum codon degeneracy permitted by the genetic code. In such cases, the nucleic acids typically hybridize under moderately stringent hybridization conditions. Exemplary “moderately stringent hybridization conditions” include a hybridization in a buffer of 40% formamide, 1 M NaCl, 1% SDS at 37° C., and a wash in 1×SSC at 45° C. A positive hybridization is at least twice background. Those of ordinary skill will readily recognize that alternative hybridization and wash conditions can be utilized to provide conditions of similar stringency.

The terms “isolated,” “purified,” or “biologically pure” refer to material that is substantially or essentially free from components that normally accompany it as found in its native state. Purity and homogeneity are typically determined using analytical chemistry techniques such as polyacrylamide gel electrophoresis or high performance liquid chromatography. A protein that is the predominant species present in a preparation is substantially purified. In particular, an isolated MET10 nucleic acid is separated from open reading frames that flank the MET10 gene and encode proteins other than MET10. The term “purified” denotes that a nucleic acid or protein gives rise to essentially one band in an electrophoretic gel. Particularly, it means that the nucleic acid or protein is at least 85% pure, more preferably at least 95% pure, and most preferably at least 99% pure.

The term “heterologous” when used with reference to portions of a nucleic acid indicates that the nucleic acid comprises two or more subsequences that are divergent from each other, which can arise naturally in the population via spontaneous mutation or genomic rearrangement, or may be artificially introduced. For instance, the nucleic acid is typically recombinantly produced, having two or more sequences from unrelated genes arranged to make a new functional nucleic acid, e.g., a promoter from one source and a coding region from another source. Similarly, a heterologous protein indicates that the protein comprises two or more subsequences that are divergent or not found in the same relationship to each other in nature (e.g., a fusion protein).

An “expression vector” is a nucleic acid construct, generated recombinantly or synthetically, with a series of specified nucleic acid elements that permit transcription of a particular nucleic acid in a host cell. The expression vector can be part of a plasmid, virus, or nucleic acid fragment. Typically, the expression vector includes a nucleic acid to be transcribed operably linked to a promoter.

The terms “polypeptide,” “peptide” and “protein” are used interchangeably herein to refer to a polymer of amino acid residues. The terms apply to amino acid polymers in which one or more amino acid residue is an artificial chemical mimetic of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers and non-naturally occurring amino acid polymer.

The term “amino acid” refers to naturally occurring and synthetic amino acids, as well as amino acid analogs and amino acid mimetics that function in a manner similar to the naturally occurring amino acids. Naturally occurring amino acids are those encoded by the genetic code, as well as those amino acids that are later modified, e.g., hydroxyproline, ÿ-carboxyglutamate, and O-phosphoserine. Amino acid analogs refers to compounds that have the same basic chemical structure as a naturally occurring amino acid, i.e., an y carbon that is bound to a hydrogen, a carboxyl group, an amino group, and an R group, e.g., homoserine, norleucine, methionine sulfoxide, methionine methyl sulfonium. Such analogs have modified R groups (e.g., norleucine) or modified peptide backbones, but retain the same basic chemical structure as a naturally occurring amino acid. Amino acid mimetics refers to chemical compounds that have a structure that is different from the general chemical structure of an amino acid, but that functions in a manner similar to a naturally occurring amino acid.

Amino acids may be referred to herein by either their commonly known three letter symbols or by the one-letter symbols recommended by the IUPAC-IUB Biochemical Nomenclature Commission. Nucleotides, likewise, may be referred to by their commonly accepted single-letter codes.

“Conservatively modified variants” applies to both amino acid and nucleic acid sequences. With respect to particular nucleic acid sequences, conservatively modified variants refers to those nucleic acids which encode identical or essentially identical amino acid sequences, or where the nucleic acid does not encode an amino acid sequence, to essentially identical sequences. Because of the degeneracy of the genetic code, a large number of functionally identical nucleic acids encode any given protein. For instance, the codons GCA, GCC, GCG and GCU all encode the amino acid alanine. Thus, at every position where an alanine is specified by a codon, the codon can be altered to any of the corresponding codons described without altering the encoded polypeptide. Such nucleic acid variations are “silent variations,” which are one species of conservatively modified variations. Every nucleic acid sequence herein which encodes a polypeptide also describes every possible silent variation of the nucleic acid. One of skill will recognize that each codon in a nucleic acid (except AUG, which is ordinarily the only codon for methionine, and TGG, which is ordinarily the only codon for tryptophan) can be modified to yield a functionally identical molecule. Accordingly, each silent variation of a nucleic acid which encodes a polypeptide is implicit in each described sequence.

As to amino acid sequences, one of skill will recognize that individual substitutions, deletions or additions to a nucleic acid, peptide, polypeptide, or protein sequence which alters, adds or deletes a single amino acid or a small percentage of amino acids in the encoded sequence is a “conservatively modified variant” where the alteration results in the substitution of an amino acid with a chemically similar amino acid. Conservative substitution tables providing functionally similar amino acids are well known in the art. Such conservatively modified variants are in addition to and do not exclude polymorphic variants, interspecies homologs, and alleles of the invention.

The following eight groups each contain amino acids that are conservative substitutions for one another:

1) Alanine (A), Glycine (G); 2) Aspartic acid (D), Glutamic acid (E); 3) Asparagine (N), Glutamine (Q); 4) Arginine (R), Lysine (K); 5) Isoleucine (I), Leucine (L), Methionine (M), Valine (V); 6) Phenylalanine (F), Tyrosine (Y), Tryptophan (W); 7) Serine (S), Threonine (T); and 8) Cysteine (C), Methionine (M) (see, e.g., Creighton, Proteins (1984)).

The terms “identical” or percent “identity,” in the context of two or more nucleic acids or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same (i.e., 60% identity, preferably 65%, 70%, 75%, 80%, 85%, 90%, or 95% identity over a specified region a region of SEQ ID NO:1), when compared and aligned for maximum correspondence over a comparison window, or designated region as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. Such sequences are then said to be “substantially identical.” This definition also refers to the compliment of a test sequence. Preferably, the identity exists over a region that is at least about 25 amino acids or nucleotides in length, or more preferably over a region that is 50-100 amino acids or nucleotides in length.

For sequence comparison, typically one sequence acts as a reference sequence, to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are entered into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. Default program parameters can be used, or alternative parameters can be designated. The sequence comparison algorithm then calculates the percent sequence identities for the test sequences relative to the reference sequence, based on the program parameters. For sequence comparison of nucleic acids and proteins to MET10 nucleic acids and proteins, the BLAST and BLAST 2.0 algorithms and the default parameters discussed below are used.

A “comparison window”, as used herein, includes reference to a segment of any one of the number of contiguous positions selected from the group consisting of from 20 to 600, usually about 50 to about 200, more usually about 100 to about 150 in which a sequence may be compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. Methods of alignment of sequences for comparison are well-known in the art. Optimal alignment of sequences for comparison can be conducted, e.g., by the local homology algorithm of Smith & Waterman, Adv. Appl. Math. 2:482 (1981), by the homology alignment algorithm of Needleman & Wunsch, J. Mol. Biol. 48:443 (1970), by the search for similarity method of Pearson & Lipman, Proc. Nat'l. Acad. Sci. USA 85:2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Dr., Madison, Wis.), or by manual alignment and visual inspection (see, e.g., Current Protocols in Molecular Biology (Ausubel et al., eds. 1995 supplement).

A preferred example of algorithm that is suitable for determining percent sequence identity and sequence similarity are the BLAST and BLAST 2.0 algorithms, which are described in Altschul et al., Nuc. Acids Res. 25:3389-3402 (1977) and Altschul et al., J. Mol. Biol. 215:403-410 (1990), respectively. BLAST and BLAST 2.0 are used, with the parameters described herein, to determine percent sequence identity for the nucleic acids and proteins of the invention. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (See, the worldwide website at ncbi.nlm.nih.gov/). This algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al., supra). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLASTN program (for nucleotide sequences) uses as defaults a word length (W) of 11, an expectation (E) of 10, M=5, N=−4 and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a word length of 3, and expectation (E) of 10, and the BLOSUM62 scoring matrix (see Henikoff & Henikoff, Proc. Natl. Acad. Sci. USA 89:10915 (1989)) alignments (B) of 50, expectation (E) of 10, M=5, N=−4, and a comparison of both strands.

The BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul, Proc. Nat'l. Acad. Sci. USA 90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.2, more preferably less than about 0.01, and most preferably less than about 0.001.

An indication that two nucleic acid sequences or polypeptides are substantially identical is that the polypeptide encoded by the first nucleic acid is immunologically cross reactive with the antibodies raised against the polypeptide encoded by the second nucleic acid, as described below. Thus, a polypeptide is typically substantially identical to a second polypeptide, for example, where the two peptides differ only by conservative substitutions. Another indication that two nucleic acid sequences are substantially identical is that the two molecules or their complements hybridize to each other under stringent conditions, as described below. Yet another indication that two nucleic acid sequences are substantially identical is that the same primers can be used to amplify the sequence.

The phrase “selectively (or specifically) hybridizes to” refers to the binding, duplexing, or hybridizing of a molecule only to a particular nucleotide sequence under stringent hybridization conditions when that sequence is present in a complex mixture (e.g., total cellular or library DNA or RNA).

By “host cell” is meant a cell that contains an expression vector and supports the replication or expression of the expression vector. Host cells may be, for example, prokaryotic cells such as E. coli or eukaryotic cells such as yeast.

III. Nucleic Acids Encoding MET10

A. General Recombinant DNA Methods

This invention relies on routine techniques in the field of recombinant and classical genetics. Generally, the nomenclature and the laboratory procedures in recombinant DNA technology described below are those well known and commonly employed in the art. Standard techniques are used for cloning, DNA and RNA isolation, amplification and purification. Generally enzymatic reactions involving DNA ligase, DNA polymerase, restriction endonucleases and the like are performed according to the manufacturer's specifications. Basic texts disclosing the general methods of use in this invention include Sambrook and Russell, Molecular Cloning, A Laboratory Manual (3d ed., Cold Spring Harbor Laboratory Press 2001); Ausubel et al., eds., Current Protocols in Molecular Biology (John Wiley & Sons 1987-2008); and Kriegler, Gene Transfer and Expression: A Laboratory Manual (1990).

For nucleic acids, sizes are given in either kilobases (kb) or base pairs (bp). These are estimates derived from agarose or acrylamide gel electrophoresis, from sequenced nucleic acids, or from published DNA sequences. For proteins, sizes are given in kilodaltons (kDa) or amino acid residue numbers. Proteins sizes are estimated from gel electrophoresis, from sequenced proteins, from derived amino acid sequences, or from published protein sequences.

Oligonucleotides that are not commercially available can be chemically synthesized according to the solid phase phosphoramidite triester method first described by Beaucage & Caruthers, Tetrahedron Letts. 22:1859-1862 (1981), using an automated synthesizer, as described in Van Devanter et. al., Nucleic Acids Res. 12:6159-6168 (1984). Purification of oligonucleotides is by either native acrylamide gel electrophoresis or by anion-exchange HPLC as described in Pearson & Reanier, J. Chrom. 255:137-149 (1983).

The sequence of the cloned genes and synthetic oligonucleotides can be verified after cloning using, e.g., the chain termination method for sequencing double-stranded templates of Wallace et al., Gene 16:21-26 (1981).

B. Cloning Methods for the Isolation of Nucleotide Sequences Encoding MET10

In general, the nucleic acid sequences encoding MET10 and related nucleic acid sequence homologues are cloned from cDNA and genomic DNA libraries or isolated using amplification techniques with oligonucleotide primers. For example, MET10 sequences are typically isolated from nucleic acid (genomic or cDNA) libraries by hybridizing with a nucleic acid probe, the sequence of which can be derived from SEQ ID NO:1, or a subsequence thereof. MET10 RNA and cDNA can be isolated from any yeast strain.

MET10 polymorphic variants, alleles, and interspecies homologues that are substantially identical to MET10 can be isolated using MET10 nucleic acid probes and oligonucleotides under stringent hybridization conditions, by screening libraries. Alternatively, expression libraries can be used to clone MET10 polymorphic variants, alleles, and interspecies homologues, by detecting expressed homologues immunologically with antisera or purified antibodies made against the core domain of MET10 which also recognize and selectively bind to the MET10 homologue.

To make a cDNA library, MET10 mRNA may be purified from any yeast strain The mRNA is then made into cDNA using reverse transcriptase, ligated into a recombinant vector, and transfected into a recombinant host for propagation, screening and cloning. Methods for making and screening cDNA libraries are well known (see, e.g., Gubler & Hoffman, Gene 25:263-269 (1983); Sambrook et al., supra; Ausubel et al., supra).

For a genomic library, the DNA is extracted from the tissue and either mechanically sheared or enzymatically digested to yield fragments of about 1-8 kb. The fragments are then separated by gradient centrifugation from undesired sizes and are constructed in bacteriophage lambda vectors. These vectors and phage are packaged in vitro. Recombinant phage are analyzed by plaque hybridization as described in Benton & Davis, Science 196:180-182 (1977). Colony hybridization is carried out as generally described in Grunstein et al., PNAS USA., 72:3961-3965 (1975).

An alternative method of isolating MET10 nucleic acids and their homologues combines the use of synthetic oligonucleotide primers and amplification of an RNA or DNA template (see U.S. Pat. Nos. 4,683,195 and 4,683,202; PCR Protocols: A Guide to Methods and Applications (Innis et al., eds, 1990)). Methods such as polymerase chain reaction (PCR) and ligase chain reaction (LCR) can be used to amplify nucleic acid sequences of MET10 directly from mRNA, from cDNA, from genomic libraries or cDNA libraries. Degenerate oligonucleotides can be designed to amplify MET10 homologues using the sequences provided herein. Restriction endonuclease sites can be incorporated into the primers. Polymerase chain reaction or other in vitro amplification methods may also be useful, for example, to clone nucleic acid sequences that code for proteins to be expressed, to make nucleic acids to use as probes for detecting the presence of MET10 encoding mRNA in physiological samples, for nucleic acid sequencing, or for other purposes. Genes amplified by the PCR reaction can be purified from agarose gels and cloned into an appropriate vector.

Amplification techniques using primers can also be used to amplify and isolate MET10 NA or RNA. For example, nucleic acids encoding MET10 or fragments thereof may be obtained by amplification of a yeast cDNA library or reverse transcribed from yeast RNA using isolated nucleic acid primer pairs having the sequences: set forth in Table 5.

These primers can be used, e.g., to amplify either the full length sequence or a probe of one to several hundred nucleotides, which is then used to screen a cDNA library for full-length MET10.

Gene expression of MET10 can also be analyzed by techniques known in the art, e.g., reverse transcription and amplification of mRNA, isolation of total RNA or poly A+ RNA, northern blotting, dot blotting, in situ hybridization, RNase protection, probing DNA microchip arrays, and the like.

Synthetic oligonucleotides can be used to construct recombinant MET10 genes for use as probes or for expression of protein. This method is performed using a series of overlapping oligonucleotides usually 40-120 by in length, representing both the sense and non-sense strands of the gene. These DNA fragments are then annealed, ligated and cloned. Alternatively, amplification techniques can be used with precise primers to amplify a specific subsequence of the MET10 gene. The specific subsequence is then ligated into an expression vector. MET10 chimeras can be made, which combine, e.g., a portion of MET10 with a portion of a heterologous MET10 to create a chimeric, functional MET10.

The gene encoding a sulfide inactive MET10 polypeptide is typically cloned into intermediate vectors before transformation into prokaryotic or eukaryotic cells for replication and/or expression. These intermediate vectors are typically prokaryote vectors, e.g., plasmids, or shuttle vectors. Isolated nucleic acids encoding sulfide inactive MET10 proteins comprise a nucleic acid sequence encoding a sulfide inactive MET10 protein and subsequences, interspecies homologues, alleles and polymorphic variants thereof. In some embodiments, the isolated nucleic acid encoding a sulfide inactive MET10 protein is SEQ ID NO:1 or a complement thereof.

C. Expression of a Sulfide Inactive MET10 Polypeptide

To obtain high level expression of a cloned gene, such as those cDNAs encoding a sulfide inactive MET10 polypeptide, one typically subclones a nucleic acid sequence of a sulfide inactive MET10 into an expression vector that contains a strong promoter to direct transcription, a transcription/translation terminator, and if for a nucleic acid encoding a protein, a ribosome binding site for translational initiation. Suitable bacterial promoters are well known in the art and described, e.g., in Sambrook et al. and Ausubel et al. Bacterial expression systems for expressing the sulfide inactive MET10 protein are available in, e.g., E. coli, Bacillus sp., and Salmonella (Palva et al., Gene 22:229-235 (1983); Mosbach et al., Nature 302:543-545 (1983). Kits for such expression systems are commercially available. Eukaryotic expression systems for mammalian cells, yeast, and insect cells are well known in the art and are also commercially available.

The promoter used to direct expression of a heterologous nucleic acid depends on the particular application. The promoter is preferably positioned about the same distance from the heterologous transcription start site as it is from the transcription start site in its natural setting. As is known in the art, however, some variation in this distance can be accommodated without loss of promoter function.

In addition to the promoter, the expression vector typically contains a transcription unit or expression cassette that contains all the additional elements required for the expression of the sulfide inactive MET10 encoding nucleic acid in host cells. A typical expression cassette thus contains a promoter operably linked to the nucleic acid sequence encoding a sulfide inactive MET10 and signals required for efficient polyadenylation of the transcript, ribosome binding sites, and translation termination. Additional elements of the cassette may include enhancers and, if genomic DNA is used as the structural gene, introns with functional splice donor and acceptor sites.

In addition to a promoter sequence, the expression cassette should also contain a transcription termination region downstream of the structural gene to provide for efficient termination. The termination region may be obtained from the same gene as the promoter sequence or may be obtained from different genes.

The particular expression vector used to transport the genetic information into the cell is not particularly critical. Any of the conventional vectors used for expression in eukaryotic or prokaryotic cells may be used. Standard bacterial expression vectors include plasmids such as pBR322 based plasmids, pSKF, pET23D, and fusion expression systems such as GST and LacZ. Epitope tags can also be added to recombinant proteins to provide convenient methods of isolation, e.g., c-myc.

Expression vectors containing regulatory elements from eukaryotic viruses are typically used in eukaryotic expression vectors, e.g., SV40 vectors, papilloma virus vectors, and vectors derived from Epstein-Barr virus. Other exemplary eukaryotic vectors include pMSG, pAV009/A+, pMTO10/A+, pMAMneo-5, baculovirus pDSVE, and any other vector allowing expression of proteins under the direction of the SV40 early promoter, SV40 later promoter, metallothionein promoter, murine mammary tumor virus promoter, Rous sarcoma virus promoter, polyhedrin promoter, or other promoters shown effective for expression in eukaryotic cells.

Some expression systems have markers that provide gene amplification such as thymidine kinase, hygromycin B phosphotransferase, and dihydrofolate reductase.

The elements that are typically included in expression vectors also include a replicon that functions in E. coli, a gene encoding antibiotic resistance to permit selection of bacteria that harbor recombinant plasmids, and unique restriction sites in nonessential regions of the plasmid to allow insertion of eukaryotic sequences. The particular antibiotic resistance gene chosen is not critical, any of the many resistance genes known in the art are suitable. The prokaryotic sequences are preferably chosen such that they do not interfere with the replication of the DNA in eukaryotic cells, if necessary.

D. Host Cells and Methods of Their Production

The invention also provides host cells that produce no hydrogen sulfide or low levels of hydrogen sulfide and express an exogenous sulfide inactive MET10 polypeptide, as described herein. An exogenous polynucleotide encoding a sulfide inactive MET10 polypeptide, wherein the amino acid at position 662 is not a threonine or a serine, is introduced into the parental host cell by methods known in the art, e.g., using recombinant or genetic crossing methods. In some embodiments, the host cells do not also express a sulfide active MET10 polypeptide, i.e., because the coding sequence for the active MET10 polypeptide has been knocked out and replaced with the coding sequence for a sulfide inactive MET10 polypeptide.

Host cells that produce low or decreased or reduced levels of hydrogen sulfide produce 50% or less H₂S in comparison to the parent strain before introducing the nucleic acid encoding the sulfide inactive MET10 polypeptide. In some embodiments, host cells that produce low or decreased levels of hydrogen sulfide produce 40%, 30%, 25%, 20%, or less H₂S in comparison to the parent strain before introducing the nucleic acid encoding the sulfide inactive MET10 polypeptide.

The host cells can be, for example, eukaryotic or prokaryotic. The host cells can be bacterial, mammalian, yeast or insect cells. In some embodiments the host cell is a yeast cell, for example, a S. cerevisiae, Kluyveromyces lactis, Yarowwia lipolytica, or Schizosaccharomyces pombe yeast cell. Yeast cells used in the production of fermented beverages, e.g., wine, port, madiera, beer, champagne, etc. (e.g., “wine yeast,” “beer yeast,” “champagne yeast,” etc.) find use for the introduction of a nucleic acid encoding an exogenous sulfide inactive MET10 polypeptide. Yeast cell strains for use in making fermented beverages, and which are candidates for MET10 inactivation (i.e., they are hydrogen sulfide producers), are commercially available from numerous sources, including without limitation, Lallemand (Lalvin) (Petaluma, Calif.; on the web at lallemandwine.us/products/yeast_chart.php) Red Star (on the web at www.redstaryeast.net/), White Labs (Boulder, Colo.; on the web at whitelabs.com/yeast search.html), Wyeast (Odell, Oreg.; on the web at wyeastlab.com), Kitzinger's, J. Laffort, Vierka, Gervin, SB Active, Unican, Siebel Inst., and Fermentis (on the web at fermentis.com/FO/EN/00-Home/10-10_home.asp). See, e.g., the worldwide web at winemaking.jackkeller.net/strains.asp for a representative list of wine and champagne yeast strains and at byo.com/referenceguide/yeaststrains/ for a representative list of beer yeast strains.

In some embodiments, the yeast cell strain is a S. cerevisiae strain. In some embodiments, the S. cerevisiae yeast cell strain is a wine yeast, for example, selected from Prise de Mousse, Premier Cuveé, French Red, Montachet, Lallemand K1, Bordeaux, UCD522, UCD940, Ba25, Ba126, Ba137, Ba220, Bb23, Bb25, Ba30, Bb32, Bb19 and Bb22. See, e.g., U.S. Pat. No. 6,140,108, the entire disclosure of which is hereby incorporated herein by reference for all purposes. Additional yeast strains that are candidates for MET10 inactivation, i.e., for the introduction of a nucleic acid encoding a sulfide inactive MET10 polypeptide, are listed in Tables 1, 3 and 4.

Standard transfection methods are used to produce bacterial, mammalian, yeast or insect cell lines that express large quantities of a sulfide inactive MET10 protein, which are then purified using standard techniques (see, e.g., Colley et al., J. Biol. Chem. 264:17619-17622 (1989); Guide to Protein Purification, in Methods in Enzymology, vol. 182 (Deutscher, ed., 1990)). Transformation of eukaryotic and prokaryotic cells are performed according to standard techniques (see, e.g., Morrison, J. Bact. 132:349-351 (1977); Clark-Curtiss & Curtiss, Methods in Enzymology 101:347-362 (Wu et al., eds, 1983).

Any of the well known procedures for introducing foreign nucleotide sequences into host cells may be used. These include the use of calcium phosphate transfection, polybrene, protoplast fusion, electroporation, liposomes, microinjection, plasma vectors, viral vectors and any of the other well known methods for introducing cloned genomic DNA, cDNA, synthetic DNA or other foreign genetic material into a host cell (see, e.g., Sambrook et al., supra). It is only necessary that the particular genetic engineering procedure used be capable of successfully introducing at least one gene into the host cell capable of expressing sulfide inactive MET10. Host cells improved to produce no hydrogen sulfide (i.e., null H₂S producers), will generally also have the active MET10 knocked-out, replaced or mutated. For example, the nucleic acid encoding a sulfide active MET10 in the parent strain can be mutated at the codon (nucleic acid positions 1984-1986) encoding the amino acid at position 662 so that this codon does not encode a threonine (or a serine). Homologous recombination techniques also find use in replacing a nucleic acid encoding a sulfide active MET10 polypeptide with a nucleic acid sequence encoding a sulfide inactive MET10 polypeptide, as described herein. See, e.g., FIG. 3 and Baudin, et al., Nucleic Acids Res (1993) 21(14):3329.

After the expression vector is introduced into the cells, the transfected cells are cultured under conditions favoring expression of sulfide inactive MET10, which is recovered from the culture using standard techniques identified below.

An exogenous MET10 nucleic acid encoding the inactive enzyme can also be transferred into novel genetic backgrounds using classical yeast genetic technologies of spore isolation, mating of spores of the opposite mating type, and isolation of the resulting diploid strains. Several rounds of genetic crosses may be used to isolate the novel MET10 allele in a different strain background. Recombinant tools need not be used for the creation of the modified strains. Exemplified methods for introducing a nucleic acid encoding a sulfide inactive MET10 polypeptide into a yeast host cell using classical yeast genetic technologies are described, for example, in U.S. Pat. No. 6,140,108.

E. Purification of MET10 Protein

Either naturally occurring or recombinant MET10 protein can be purified for use in functional assays. Naturally occurring MET10 proteins are purified, e.g., from yeast and any other source of a MET10 homologue. Recombinant MET10 is purified from any suitable expression system.

MET10 may be purified to substantial purity by standard techniques, including selective precipitation with such substances as ammonium sulfate; column chromatography, immunopurification methods, and others (see, e.g., Scopes, Protein Purification: Principles and Practice (1982); U.S. Pat. No. 4,673,641; Ausubel et al., supra; and Sambrook et al., supra).

A number of procedures can be employed when recombinant MET10 protein is being purified. For example, proteins having established molecular adhesion properties can be reversibly fused to MET10. With the appropriate ligand, MET10 can be selectively adsorbed to a purification column and then freed from the column in a relatively pure form. The fused protein is then removed by enzymatic activity. Finally, MET10 could be purified using immunoaffinity columns.

IV. Determining whether a Yeast Strain will Produce H₂S by Detecting MET10 Nucleic Acid Sequences

In one embodiment of the invention, methods of determining whether a particular yeast strain is an H₂S producer are provided. According to the methods of the invention, the MET10 allele of the yeast strain is analyzed and compared to the MET10 alleles disclosed herein to determine whether the yeast strain is a high, low, or non-producer of H₂S. Determination of the presence or absence of a particular MET10 allele is generally performed by analyzing a nucleic acid sample that is obtained from a yeast (e.g., of the genus Saccharomyces) to be analyzed. Often, the nucleic acid sample comprises genomic DNA. It is also possible to analyze RNA samples for the presence of MET10 alleles.

Detection techniques for evaluating nucleic acids for the presence of a single base change involve procedures well known in the field of molecular genetics. Further, many of the methods involve amplification of nucleic acids. Ample guidance for performing the methods is provided in the art. Exemplary references include manuals such as PCR Technology: PRINCIPLES AND APPLICATIONS FOR DNA AMPLIFICATION (ed. H. A. Erlich, Freeman Press, NY, N.Y., 1992); PCR PROTOCOLS: A GUIDE TO METHODS AND APPLICATIONS (eds. Innis, et al., Academic Press, San Diego, Calif., 1990); CURRENT PROTOCOLS IN MOLECULAR BIOLOGY, Ausubel, 1994-2008, Wiley Interscience, including supplemental updates through April 2004; Sambrook & Russell, Molecular Cloning, A Laboratory Manual (3rd Ed, 2001).

Methods for detecting single base changes well known in the art often entail one of several general protocols: hybridization using sequence-specific oligonucleotides, primer extension, sequence-specific ligation, sequencing, or electrophoretic separation techniques, e.g., singled-stranded conformational polymorphism (SSCP) and heteroduplex analysis. Exemplary assays include 5′ nuclease assays, template-directed dye-terminator incorporation, molecular beacon allele-specific oligonucleotide assays, single-base extension assays, and SNP scoring by real-time pyrophosphate sequences. Analysis of amplified sequences can be performed using various technologies such as microchips, fluorescence polarization assays, and matrix-assisted laser desorption ionization (MALDI) mass spectrometry. In addition to these frequently used methodologies for analysis of nucleic acid samples to detect single base changes, any method known in the art can be used to detect the presence of the MET10 mutations described herein.

Although the methods typically employ PCR steps, other amplification protocols may also be used. Suitable amplification methods include ligase chain reaction (see, e.g., Wu & Wallace, Genomics 4:560-569, 1988); strand displacement assay (see, e.g., Walker et al., Proc. Natl. Acad. Sci. USA 89:392-396, 1992; U.S. Pat. No. 5,455,166); and several transcription-based amplification systems, including the methods described in U.S. Pat. Nos. 5,437,990; 5,409,818; and 5,399,491; the transcription amplification system (TAS) (Kwoh et al., Proc. Natl. Acad. Sci. USA 86:1173-1177, 1989); and self-sustained sequence replication (3SR) (Guatelli et al., Proc. Natl. Acad. Sci. USA 87:1874-1878, 1990; WO 92/08800). Alternatively, methods that amplify the probe to detectable levels can be used, such as Qβ-replicase amplification (Kramer & Lizardi, Nature 339:401-402, 1989; Lomeli et al., Clin. Chem. 35:1826-1831, 1989). A review of known amplification methods is provided, for example, by Abramson and Myers in Current Opinion in Biotechnology 4:41-47, 1993.

In some embodiments, the MET10 allele is detected using oligonucleotide primers and/or probes. Oligonucleotides can be prepared by any suitable method, including chemical synthesis. Oligonucleotides can be synthesized using commercially available reagents and instruments. Alternatively, they can be purchased through commercial sources. Methods of synthesizing oligonucleotides are well known in the art (see, e.g., Narang et al., Meth. Enzymol. 68:90-99, 1979; Brown et al., Meth. Enzymol. 68:109-151, 1979; Beaucage et al., Tetrahedron Lett. 22:1859-1862, 1981; and the solid support method of U.S. Pat. No. 4,458,066).

A. PCR Identification of MET10 Alleles

In some embodiments, PCR is used to amplify nucleic acids encoding MET10 alleles. A general overview of the applicable technology can be found in PCR Protocols: A Guide to Methods and Applications (Innis et al. eds. (1990)) and PCR Technology: Principles and Applications for DNA Amplification (Erlich, ed. (1992)). In addition, amplification technology is described in U.S. Pat. Nos. 4,683,195 and 4,683,202.

PCR permits the copying, and resultant amplification of a target nucleic acid, e.g., a nucleic acid encoding MET10. Briefly, a target nucleic acid, e.g. DNA from a sample comprising yeast strains of interest, is combined with a sense and antisense primers, dNTPs, DNA polymerase and other reaction components. (See, Innis et al., supra) The sense primer can anneal to the antisense strand of a DNA sequence of interest. The antisense primer can anneal to the sense strand of the DNA sequence, downstream of the location where the sense primer anneals to the DNA target. In the first round of amplification, the DNA polymerase extends the antisense and sense primers that are annealed to the target nucleic acid. The first strands are synthesized as long strands of indiscriminate length. In the second round of amplification, the antisense and sense primers anneal to the parent target nucleic acid and to the complementary sequences on the long strands. The DNA polymerase then extends the annealed primers to form strands of discrete length that are complementary to each other. The subsequent rounds serve to predominantly amplify the DNA molecules of the discrete length.

In general, PCR and other methods of amplification use primers which anneal to either end of the DNA of interest. For example, nucleic acids encoding MET10 alleles or fragments thereof may be amplified using isolated nucleic acid primer pairs having the sequences set forth in Table 5.

B. Detection of Amplified Products

Amplified products can be detected using any means known in the art, including, e.g., restriction fragment length polymorphism (RFLP) analysis; denaturing gel electrophoresis (see, e.g., Erlich, ed., PCR TECHNOLOGY, PRINCIPLES AND APPLICATIONS FOR DNA AMPLIFICATION, W. H. Freeman and Co, New York, 1992, Chapter 7), direct sequencing, and HPLC-based analysis. Suitable sequence methods include e.g., dideoxy sequencing-based methods and Maxam and Gilbert sequence (see, e.g., Sambrook and Russell, supra). Suitable HPLC-based analyses include, e.g., denaturing HPLC (dHPLC) as described in e.g., Premstaller and Oefner, LC-GC Europe 1-9 (July 2002); Bennet et al., BMC Genetics 2:17 (2001); Schrimi et al., Biotechniques 28(4):740 (2000); and Nairz et al., PNAS USA 99(16):10575-10580 (2002); and ion-pair reversed phase HPLC-electrospray ionization mass spectrometry (ICEMS) as described in e.g., Oberacher et al.; Hum. Mutat. 21(1):86 (2003). Other methods for characterizing single base changes in MET10 alleles include, e.g., single base extensions (see, e.g., Kobayashi et al, Mol. Cell. Probes, 9:175-182, 1995); single-strand conformation polymorphism analysis, as described, e.g., in Orita et al., Proc. Nat. Acad. Sci. 86, 2766-2770 (1989), allele specific oligonucleotide hybridization (ASO) (e.g., Stoneking et al., Am. J. Hum. Genet. 48:70-382, 1991; Saiki et al., Nature 324, 163-166, 1986; EP 235,726; and WO 89/11548); and sequence-specific amplification or primer extension methods as described in, for example, WO 93/22456; U.S. Pat. Nos. 5,137,806; 5,595,890; 5,639,611; and U.S. Pat. No. 4,851,331; 5′-nuclease assays, as described in U.S. Pat. Nos. 5,210,015; 5,487,972; and 5,804,375; and Holland et al., 1988, Proc. Natl. Acad. Sci. USA 88:7276-7280.

V. Methods for Reducing H₂S Levels in Fermented Beverages

Yeast strains comprising the MET10 nucleic sequences described herein can be used to reduce H₂S levels in fermented beverages (e.g., wine and beer).

According to the methods of the invention, yeast cells transformed with an exogenous nucleic acid sequence encoding a sulfide inactive MET10 polypeptide, as described herein, are contacted with a fermentation medium (e.g., a must or a wort) and the mixture is incubated for a suitable amount of time (e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14 days) in a suitable first fermentation vessel (e.g., a tank, barrel, crock, jar, pail or polyethylene container) at a suitable temperature (e.g., about 70-75° F.) for fermentation to proceed. The liquid may then be transferred to a second fermentation vessel. The second vessel may or may not be sealed and the contents are incubated for a suitable amount of time (e.g., 2, 3, 4, 5, 6, 7, or 8 weeks) at a suitable temperature (e.g., about 60-65° F.) for anaerobic fermentation and aging to proceed. The liquid is then transferred to a third vessel for racking (i.e., clarification). The third vessel is sealed and sediment is allowed to settle for a suitable amount of time (e.g., 2, 3, 4, 5, 6, 7, or 8 weeks). Racking may be repeated one, two, three or more times prior to bottling the fermented beverage. The native MET10 allele may be replaced either using recombinant DNA technologies or crossed in through classical breeding strategies. The UCD932 MET10 allele confers a white colony color on BiGGY agar, allowing this allele to be followed in genetic crosses and to be readily screened during production to demonstrate successful implantation of the strain.

When the wine is clear and all fermentation and pre-bottle aging has stopped, siphon into wine bottles and cork the bottles securely. Leave corked bottles upright for 3-5 days and then store them on their side at 55 degrees Fahrenheit for six months (white wine) to a year (red wine) before sampling. If not up to expectations, allow to age another year or more.

The yeast may be transformed using any method know in the art including, e.g., Liac/SS carrier DNA/PEG method described by Gietz and Woods Methods in Enzymology 350: 87-96 (2002); Agatep et al., Technical Tips Online Transformation of Saccharomyces cerevisiae by the lithium acetate/single-stranded carrier DNA/polyethylene glycol (LiAc/ss-DNA/PEG) protocol (1998); and the yeast two hybrid method described in Gietz et al., Mol Cell Biochem 172:67-79 (1997). Methods for preparing yeast cells that are competent for transformation are set forth in, e.g., Dohmen et al. (1991) Yeast 7: 691-692.

VI. Kits

MET10 and its homologues are useful tools for more specific and sensitive identification of yeast strains that are low H₂S producers. For example, nucleic acids that specifically hybridize to MET10 nucleic acids, such as MET10 probes and primers (e.g., as set forth in Table 5), MET10 nucleic acids (e.g. as set forth in FIG. 2), are used to identify yeast strains that are low H₂S producers.

The invention also provides kits and solutions for detecting the MET10 alleles described herein. For example, the invention provides kits that include one or more reaction vessels that have aliquots of some or all of the reaction components of the invention in them. Aliquots can be in liquid or dried form. Reaction vessels can include sample processing cartridges or other vessels that allow for the containment, processing and/or amplification of samples in the same vessel. Such kits allow for ready detection of amplification products of the invention into standard or portable amplification devices. The kits can also include written instructions for the use of the kit to amplify and control for amplification of a target sample.

Kits can include, for instance, amplification reagents comprising primers sufficient to amplify at least one MET10 allele, and at least one probe for amplifying and detecting the polynucleotide sequence. In addition, the kit can include nucleotides (e.g., A, C, G and T), a DNA polymerase and appropriate buffers, salts and other reagents to facilitate amplification reactions.

In some embodiments, the kits comprise vessels such as sample processing cartridges useful for rapid amplification of a sample as described in Belgrader, et al., Biosensors and Bioelectronics 14:849-852 (2000); Belgrader, et al., Science, 284:449-450 (1999); and Northrup, M. A., et al. “A New Generation of PCR Instruments and Nucleic Acid Concentration Systems” in PCR PROTOCOLS (Sninsky, J. J. et al (eds.)) Academic, San Diego, Chapter 8 (1998)).

EXAMPLES

The following examples are offered to illustrate, but not to limit the claimed invention.

Example 1 Identification of Genes Affecting H₂S Production

To better understand the mechanisms and pathways through which H₂S is formed, and to develop future prevention or management strategies, a screen of the yeast deletion strain set, comprised of 4,827 mutants, was performed to identify genes affecting H₂S production. A collection of native isolates of wine fermentations (Mortimer 1994) was screened in order to define the basis of the bias of colony color versus actual H₂S production. In addition, a yeast null mutant collection whose parental strain is a non-H₂S producer was screened for genes that when mutated resulted in elevated H₂S formation. The possible additive effects on H₂S formation of these mutations were also evaluated.

Materials and Methods

Yeast strains and culture conditions. The yeast strains used for this study and whose results are presented are listed in Table 1. Yeast strains were maintained and grown on yeast extract peptone dextrose medium with 2% glucose (YPD) (Sherman et al. 1974). The same medium (YPD) with geneticin (G418, 0.2 mg/ml) was used for maintenance of deletion strains carrying the G418^(R) marker.

TABLE 1 Native and Industrial Yeast Strains Strains Known genotypes or descriptions Reference or Source UCD522 Industrial isolates UCD UCD713 Industrial isolates UCD UCD819 Industrial isolates UCD UCD932 (Ba2) Native isolates UCD UCD933 Native isolates UCD UCD934 (Ba25) Native isolates UCD UCD935 Native isolates UCD UCD936 Native isolates UCD UCD937 Native isolates UCD UCD938 (Ba86) Native isolates UCD UCD939 (Ba99) Native isolates UCD UCD940 (Ba111) Native isolates UCD UCD941 Native isolates UCD UCD942 (Ba126) Native isolates UCD UCD943 Native isolates UCD UCD944 Native isolates UCD UCD945 Native isolates UCD UCD946 Native isolates UCD UCD947 Native isolates UCD UCD948 Native isolates UCD UCD949 Native isolates UCD UCD950 (Ba196) Native isolates UCD UCD951 Native isolates UCD UCD952 Native isolates UCD UCD953 Native isolates UCD UCD954 Native isolates UCD UCD955 Native isolates UCD UCD956 (Ba224) Native isolates UCD UCD957 (Ba229) Native isolates UCD UCD958 Native isolates UCD YLR303W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0met17::G418 Open Biosystems YGR155W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0cys4::G418 Open Biosystems YHL031C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δgos1::G418 Open Biosystems YER060W-A BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0fcy22::G418 Open Biosystems YGR138C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0tpo2::G418 Open Biosystems YDR158W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0hom2::G418 Open Biosystems YJR139C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0hom6::G418 Open Biosystems YNL315C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0atp11::G418 Open Biosystems YIL074C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0ser33::G418 Open Biosystems YNL031C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0hht2::G418 Open Biosystems YBR095C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0rxt2::G418 Open Biosystems YLR384C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0iki3::G418 Open Biosystems YPL035C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0ypl035c::G418 Open Biosystems YDL047W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0sit4::G418 Open Biosystems YBL046W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0psy4::G418 Open Biosystems YGL029W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0cgr1::G418 Open Biosystems

DNA and Genetic Manipulations. Genetic manipulations including crosses, sporulation and tetrad analysis were carried out using standard procedures (Gunthrie 1991). Gene deletions were confirmed by PCR using the upstream forward primer and an internal reverse primer to the KanMX disruption marker—JKKanRE. Amplification conditions were as follows: 30 cycles of 94° C. for 2 min., 92° C. for 45 s, 56° C. for 30 s, 72° C. for 1 min and a final extension at 72° C. for 7 min. Primer sequences are listed in Table 5.

Screen of deletion set and native strains. The deletion set (Open Biosystems, Huntsville, Ala.) and collection of native isolates were screened on BiGGY agar, a bismuth glucose glycine yeast agar (Nickerson 1953). They were also screened in synthetic grape juice medium “Minimal Must Media” (MMM) (Spiropoulos et al. 2000) initially with 123 mg of nitrogen equivalents/liter. The nitrogen level was generated using 0.2 g of L-arginine/liter and 0.5 g of ammonium phosphate/liter.

Analysis of hydrogen sulfide formation. Hydrogen sulfide production was evaluated using the lead acetate method (Spiropoulos et al. 2000; Giudici, P., and R. E. Kunkee, 1994). Hydrogen sulfide formation was initially detected using paper strips (2×10 cm, 3 mm Whatman filter paper) which had been previously treated with 50 μL of 5% lead acetate solution and allowed to dry at room temperature. The lead acetate strips were folded in half and inserted into 50 mL culture tubes with the culture tube cap securing either end of the strip, enclosing the mid-portion of the lead acetate strip in the gaseous environment over the liquid medium. Hydrogen sulfide formation was qualitatively measured by degree of blackening of the lead acetate strip. This screen was conducted by Carrie Findleton as part of her MS thesis dissertation.

All positives were confirmed using a more sensitive and semi-quantitative method. A Whatman filter paper strip (1.5×8.0 cm, 3 mm) was rolled and placed in a 1 ml bulb-less plastic pipette and treated with 250 μl of a 3% lead acetate solution. The paper was allowed to dry at room temperature and the plastic lead acetate column was then attached to the 50 mL culture tube with a silicone stopper. Hydrogen sulfide formation was measured by mm of darkening on the paper.

In subsequent experiments, to quantify H₂S production, packed lead acetate columns were used, in which each mm of blackening on the column denoted 4 μg/L H₂S. Lead acetate columns were purchased from Figasa International Inc. (Seoul, Korea).

Fermentation conditions. To identify yeast strains and nutritional conditions impacting in hydrogen sulfide formation, yeast cultures were grown in 5 mL of modified Triple M Medium in 50 mL culture tubes at 25° C. on shaker tables at 120 rpm. The synthetic grape juice medium “Minimal Must Medium” (MMM) (Giudici et al. 1993) was used and modified from the original recipe to produce seven different nitrogen and micronutrient compositions. Arginine, ammonium phosphate, and Casamino acids additions were manipulated to adjust nitrogen concentration, and YNB (Yeast Nitrogen Base without Amino Acids and Ammonium Sulfate) additions were adjusted to control for nutrient and vitamin concentration. Triple M modifications are illustrated in Table 2.

TABLE 2 Modified Triple M Medium Composition Ammonium Casamino Arginine Phosphate acids YNB MMM Variety (g/liter) (g/liter) (g/liter) (g/liter) 433 g nitrogen equivalents/liter 0.8 1 2 1.7 123 g nitrogen equivalents/liter 0.2 0.1 2 1.7 123 g nitrogen equivalents/liter and ⅕ YNB 0.2 0.1 2 0.34 65 g nitrogen equivalents/liter, no Casamino acids 0.2 0.03 0 1.7 65 g nitrogen equivalents/liter 0.107 0.015 1 1.7 65 g nitrogen equivalents/liter and ½ YNB 0.107 0.015 1 0.85 65 g nitrogen equivalents/liter and ⅓ YNB 0.107 0.015 1 0.567

Yeast inocula were obtained from plated yeast colonies. This procedure may have resulted in some variation in cell number at inoculation, but was necessary due to the large number of yeast strains involved in the preliminary screening process. Hydrogen sulfide formation was evaluated after four days by degree of blackening of the lead acetate strip. Strains that did not grow in four days were repeated to insure there was no other variable that resulted in the absence of growth.

For selected strains of interest, hydrogen sulfide formation was quantified using lead acetate columns. For this purpose, fermentations were conducted in 500-mL Erlenmeyer flasks, containing 300 mL MMM, with a lead acetate column secured to the top of the flask in a rubber stopper. For this purpose, 123 mg/L nitrogen MMM was used to more accurately emulate low nitrogen grape juice conditions. Fermentations were initiated at a density of 1.33×10⁵ cells/ml by inoculation with stationary-phase cells from a culture pre-grown in Triple M Medium of the same composition. The fermentations performed in triplicate, incubated at 25° C. and 120 rpm, and monitored over seven days by weight loss and darkening on the lead acetate column.

Screening of the deletion set and native isolates on BiGGY agar. To assess the H₂S production of the deletion strains and native isolates they were initially all plated on BiGGY agar and the color of the colonies evaluated. The colony colors were white, light tan, tan (deletion set parental strain color), light brown, brown or black (Linderholm et al. 2006). From the deletion set, four colonies were white, 258 were light tan, 4478 were tan, 59 were light brown, 28 were brown and one colony was black ranging in colony color from light to dark.

Screening of native and commercial isolates in synthetic juice. Thirty native isolates were screened in synthetic juice MMM with 123 mg/L nitrogen to evaluate H₂S production versus colony color. Non-H₂S-producers (i.e., UCD932, UCD713 UCD819, UCD938, UCD942, UCD954 and UCD956) had colony colors ranging from white to light brown. Strains producing H₂S ranged from light tan (3) to tan (10) to light brown (5) to brown (5). The darkest colonies (brown) ranged from 2-6 mm of H₂S and are in the mid range of production. The three highest producers (over 10 mm) are light tan, tan and light brown on BiGGY. Native isolates on BiGGY and in MAIM are shown in Table 3.

TABLE 3 Native isolates on BiGGY and in MMM Strain Colony color H₂S (mm) UCD522 Tan 4 UCD713 Tan 0 UCD819 Tan 0 UCD932 White 0 UCD933 Brown 3 UCD934 Tan 5.5 UCD935 Tan 10.5 UCD936 Brown 2 UCD937 Light Tan Trace UCD938 Tan 0 UCD939 Light Tan 14.5 UCD940 Brown 6 UCD941 Brown 2 UCD942 Light Tan 0 UCD943 Light Brown 3.5 UCD944 Light Brown Trace UCD945 Tan 8 UCD946 Tan 2 UCD947 Tan 1.75 UCD948 Tan 2.5 UCD949 Light Tan Trace UCD950 Light Brown 19 UCD951 Tan 5.5 UCD952 Tan 8 UCD953 Light Brown Trace UCD954 Light Brown 0 UCD955 Brown 4 UCD956 White 0 UCD957 Tan 9 UCD958 Light Brown 1

Example 2 Identification of Mutations in the MET10 Allele of UCD932

As set forth in Example 1 above, UCD932 was identified as a yeast strain which produces little to undetectable hydrogen sulfide under a variety of environmental conditions. This strain also produces white colonies on BiGGY agar, associated with low sulfite reductase activity. A screen of the deletion set of strains for S. cerevisiae yielded four possible mutations resulting in white colonies, all encoding for components of sulfite reductase. Genetic crosses revealed that the white colony BiGGY phenotype in UCD932 was due to an alteration of the MET10 gene. The MET10 deletion strain was a methionine auxotroph but UCD932 is not a methionine auxotroph, indicating that sulfite reductase activity is still retained by the cell. To define the genetic basis of this low sulfide production ability, the MET10 and several other genes in the sulfate reduction pathway, identified as possibly playing a role in the suppression of H₂S in S. cerevisiae, (Linderholm et al. 2006) were sequenced. This would allow for identification of alleles that could be replaced in H₂S producing wine strains to eliminate the undesirable sulfide characteristic.

Materials and Methods

Yeast strains and culture conditions. The yeast strains used for this study are listed in Table 4. Yeast strains were maintained and grown on yeast extract peptone dextrose medium with 2% glucose (YPD) (Sherman et al. 1974). The same medium (YPD) with geneticin (G418, 0.2 mg/ml) or hygromycin (Hph, 0.3 mg/ml) were used for maintenance of deletion strains carrying the G418^(R) or HphMX marker. Minimal media (YNB) was made with 0.67% yeast nitrogen base without amino acids and supplemented with casamino acids as recommended (Sherman). Selective-met dropout media were made similar to YNB without the methionine.

TABLE 4 Additional yeast strains Strains Known genotypes or descriptions Reference or Source UCD932 (Ba2) Native isolates UCD UCD934 (Ba25) Native isolates UCD UCD938 (Ba86) Native isolates UCD UCD939 (Ba99) Native isolates UCD UCD940 (Ba111) Native isolates UCD UCD942 (Ba126) Native isolates UCD UCD950 (Ba196) Native isolates UCD UCD956 (Ba224) Native isolates UCD UCD957 (Ba229) Native isolates UCD UCD522 Industrial isolates UCD YKR069W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0met1::G418 Open Biosystems YJR137C BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0met5::G418 Open Biosystems YBR213W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0met8::G418 Open Biosystems YFR030W BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0met10::G418 Open Biosystems ALY38 UCD932 MET10^(S288C) This Study ALY39 UCD932 MET10^(UCD932) This Study ALY95 UCD932 MET10^(UCD950) This Study ALY72 BY4742 MET10^(UCD950) This Study ALY40 UCD950 MET10^(S288C) This Study ALY41 UCD950 MET10^(UCD932) This Study ALY126 UCD950 MET10^(UCD950) This Study ALY127 UCD939 MET10^(UCD939) This Study ALY128 UCD939 MET10^(S288C) This Study ALY130 UCD940 MET10^(S288C) This Study ALY129 UCD940 MET10^(UCD940) This Study ALY131 UCD940 MET10^(UCD932) This Study ALY132-1A UCD522 MET10::KanMX4 This Study ALY133-1B UCD522 MET10^(S288C) This Study ALY134-1C UCD522 MET10^(S288C) This Study ALY135-1D UCD522 MET10::KanMX4 This Study ALY136-1A UCD522 MET10^(UCD522) This Study ALY137-1B UCD522 MET10::hphNT1 This Study ALY138-1C UCD522 MET10::hphNT1 This Study ALY139-1D UCD522 MET10^(UCD522) This Study ALY140-1A UCD522 MET10^(UCD932) This Study ALY141-1B UCD522 MET10^(UCD932) This Study ALY142-1C UCD522 MET10::KanMX4 This Study ALY143-1D UCD522 MET10::KanMX4 This Study

Screen of deletion set. The yeast deletion set (Open Biosystems, Huntsville, Ala.) was screened on BiGGY agar, a bismuth glucose glycine yeast agar (Nickerson 1953), supplemented with casamino acids (Sherman 1974). Each strain was plated onto BiGGY agar and incubated at 30° C. for 48 hours. The resulting colonies were assessed for color.

Sequence Analysis. The sequence analysis of MET10, HOM2, HOM6, SER33, MET1, MET5 and MET8 were performed in 169 native and industrial strains of yeast. Chromosomal DNA was extracted from the cell pellets using the smash and grab protocol (Hoffman and Winston 1987) and amplification of the genes was carried out using High Fidelity Platinum Taq (Invitrogen, Carlsbad, Calif.) and primers PCR-MET10-F/PCR-MET10-R for MET10, HOM2-F/HOM2-R for HOM2, HOM6-F/HOM6-R for HOM6, SER33-F/SER33-R for SER33, MET1-F/MET1-R for MET1, MET5-F/MET5-R for MET5 and MET8-F/MET9-R for MET8 (Table 5). Amplification conditions were as follows: 30 cycles of 94° C. for 1 min., 94° C. for 30 s, 50° C. for 30 s, 68° C. for 4 min. and a final extension at 68° C. for 7 min.

TABLE 5 PCR Primers SEQ ID Primer Sequence (5′ → 3′) NO: HOM2 HOM2-F CACTTAAGTACACATACAAA 35 HOM2-R GGGTCAGCGAGAGAATT 36 HOM6 HOM6-F CCTGGTGGTAAAGTTGGG 37 HOM6-R GATTGTAGAAGATTGAGTAG 38 SER33 SER33-F GGAATCTCCCAGGTTTAAT 39 SER33-R GGGCAATCAAAGGCTAT 40 MET1 MET1-F CGCTAATAAACTCGCTACAAAAG 41 MET1-R CGTCCTTTTTGCTCAATATCC 42 MET5 MET5-F GCTGCAAGCAGTTATATAAAGTG 43 MET5-R AAAACCGAACTAGCCGAAG 44 MET8 MET8-F AAAATCGCTACAAAGTCCG 45 MET8-R GCATTGTTGTTCGTTCTCC 46 MET10 primers PCR-MET10-F CGGATCCCCAATCACCATAACACTT 47 PCR-MET10-R GCCGCGGTAGGGTCTTCAGGACGAG 48 MET10-F-KO CAAATAGTTTCGTTTAGATGG 49 MET10-R-KO GTATAATGTGATGGTTAGTT 50 MET10-hphMX-F ACTGTGTTTATCACTTATGGGTCTTTAGAATCC 51 GAATTGTATTTTGATGGCCGCACGG MET10-hphMX-R AACAATTCAAAAATGTCAGCATATGTATAATA 52 CTCCACATAATCGACAGCAGTATAGCGACCA Confirmation primers JKKanRE GGGCGACAGTCACATCAT 53 HYGROB ChK_R TGACGGTGTCGTCCATCAC 54

All sequencing was carried out at the College of Biological Sciences Sequencing Facility at the University of California, Davis by using an ABI 3730 capillary electrophoresis genetic analyzer and ABI BigDye Terminator version 3.1 cycle sequencing chemistry (Foster City, Calif.), primers used are listed in Table 6. Sequence data were edited and analyzed with BioEdit sequence Alignment Editor (version 5.0.9; Nucleic Acids Symp. Ser. 41:95-98).

TABLE 6 MET Sequencing Primers Primer Sequence (5′ → 3′) SEQ ID NO: MET1 MET1-S1F TGGGGAGAGTTCTGGTATGCAAG 55 MET1-S2F CAGATGGTTATCTCAGATAATGGAG 56 MET1-S3F TTTCTTCAAAGATCACGGATATATT 57 MET1-S1R GCTATATCACGTTGAGTAGCGG 58 MET1-S2R GGTACTACACCCTCTGTGACAGTT 59 MET1-S3R CTCAGTTTTTGGCATTGCCA 60 MET5 MET5-S1F CCTAATAAACTTCCATTGGTGATTA 61 MET5-S2F CCGTTTTACAGGGTGTCTCTAAGA 62 MET5-S3F GACGCGATCTTGACGAAGCT 63 MET5-S4F GAATCTGGTTACTGGCCATTGT 64 MET5-S5F CTGAAAAATGACACCGACTTGG 65 MET5-S6F TGGCTTGCTCTGGATCACTT 66 MET5-S7F CGATGTCGGTTTAGTTGCTATAGTT 67 MET5-s8F TGGTAATCAACATTTGGTTATCTCT 68 MET5-S1R GGGCAACCAGTCATTCTCATAA 69 MET5-S2R CTTCGACACCCATATCATCTACAG 70 MET5-S3R CAATTTTCCCATATCAGCGA 71 MET5-S4R CATCATCAACAGCAGCGCCG 72 MET5-S5R CTGATCGAAGGCAGCCTTGC 73 MET5-S6R CATATGGCTCTGAATCAATCAATAA 74 MET5-S7R TTCACAACTTTTTTGACAGAAGAA 75 MET5-S8R CGTTAGCAATCTCCAAGGTAGGAA 76 MET8 MET8-S1F GCAGTGACTTCAAAGACGAATACC 77 MET8-S2F CTGGAGGACGCTGTCGTCAA 78 MET8-S1R TCATCTCTTACTAGAGCGCCAA 79 MET8-S2R GGTCCCAGTTCGGATTGATAA 80 MET10 MET10 SEQ1-F AGTCATCTTCGAGCAAA 81 MET10 SEQ2-F TCATGATGGTAAGTTTC 82 MET10 SEQ3-F TCAACGTCAGAGTGCCATT 83 MET10 SEQ4-F ATCAGTCGTTGAAGATGTC 84 MET10 SEQ5-F CTGAGATCTCTGATATTGC 85 MET10 SEQ6-F TGCAGTAGATTTGAAGAGAT 86 MET10 SEQ7-F CACACACATCGGCGCT 87 MET10 SEQ1-R CGGAGTCACGACACCAT 88 MET10 SEQ2-R GGCTGAAACTTGAGATCTC 89 MET10 SEQ3-R CTTGACGTAACTTTCTACAG 90 MET10 SEQ4-R TCATAATCAGCAGGCGTAAC 91 MET10 SEQ5-R CTTCTCTTCAATGGTTCAAT 92 MET10 SEQ6-R AGTAGGGCCAGACAAGT 93

GenBank Accession numbers for these sequences are: UCD932 MET10 (EF058164), UCD938 MET10 (EF058165), UCD939 MET10 (EF058166), UCD940 MET10 (EF058167), UCD942 MET10 (EF058168), UCD956 (EF058169), UCD522 MET10 (EF058170), UCD957 MET10 (EF058171), UCD934 MET10 (EF058172), UCD950MET10 (EF058173), UCD932 SER33 (EF058174), UCD939 SER33 (EF058175), UCD940 SER33 (EF058176), UCD956 SER33 (EF058177), UCD950 SER33 (EF058178), UCD932 HOME (EF058179), UCD932 MET1 (EF058180), UCD939 MET1 (EF058181), UCD940 MET1 (EF058182), UCD950 MET1 (EF058183), UCD956 MET1 (EF058184), UCD956 MET5 (EF058185), UCD932 MET5 (EF058186), UCD940 MET5 (EF058187), UCD939 MET5 (EF058188), UCD932 MET8 (EF058189), UCD939 MET8 (EF058190), UCD940 MET8 (EF058191), UCD950MET8 (EF058192), UCD956MET8 (EF058193).

Genetic Manipulations. Genetic manipulations including crosses, sporulation and tetrad analysis were carried out using standard procedures (Guthrie 1991).

Plasmids, DNA manipulations, and transformation methods. The plasmids pAL51 (MET10^(S288C)), pAL52 (MET10^(UCD932)) were used in this study. Primers, PCR-MET10-F/PCR-MET10-R (Table 5), carrying the restriction sites BamHI and SacII were designed to amplify MET10 from yeast strain UCD932 and S288C chromosomal DNA (Invitrogen, Carlsbad, Calif.). Plasmid pYC130 (Olesen et al. 2000), is a centromeric vector carrying the selectable marker G418^(R) was digested with BamHI and SacII (New England Biolabs, Ipswich, Mass.) to allow for the ligation of MET10. The resulting plasmids, pAL51 (MET10^(S288C)), pAL52 (MET10^(UCD932)) were used for transformation. Gene deletions of MET10 were created using a PCR-based technique, FIG. 3 (Baudin 1993). A KanMX containing deletion cassette (Yeast Deletion collection) with overhangs of non-coding regions on either side of MET10 was PCR amplified using primers, MET10-F-KO/MET10-R-KO, and the linear PCR fragment was transformed into yeast diploid strains UCD522, UCD932, UCD939, UCD940 and UCD950. By homologous recombination one copy of the intact MET10 was replaced with the knockout cassette generating strains carrying a copy of both an intact copy of MET10 and a KanMX marker. All of the strains, except UCD522MET10/KanMX, were then sporulated and those homologous for G418^(R) were used for further experiments. Gene deletions were confirmed by PCR using the upstream forward primer and an internal reverse primer to the KanMX disruption marker—JKKanRE.

To knockout the remaining intact copy of MET10 in UCD522 MET10/KanMX, a HphMX cassette was amplified from BamHI linearized pYC140 (Hansen et al. 2003) using primers MET10-hphMX-F/MET10-hphMX-R, and the linear PCR fragment was transformed into ALY29. A methionine auxotrophic colony displaying both G418^(R) and Hph^(R) was selected and the HphMX deletion confirmed by PCR using the upstream forward primer and an internal reverse primer to the HphMX disruption marker—HYGROB CHK_R.

Allele swaps of MET10 were also created using a PCR-based technique (FIG. 3) (Baudin 1993). Alleles of MET10 were amplified from S288C, UCD932, UCD939, UCD940, UCD950 and UCD522 using primers MET10-F-KO/MET10-R-KO. The linear PCR fragments amplified from S288C and UCD932 were then transformed into the methionine auxotrophic strains. The other fragments were transformed into individual strains to create the corresponding control strains.

Strains displaying ability to grow on methionine auxotrophic plates were selected and sporulated to create strains homologous for MET10 for further experiments. S. cerevisiae was transformed using the lithium acetate method adapted from the Schiestl and Gietz (1989) and E. coli was transformed using the method described by Inoue et al. (Inoue et al. 1990). E. coli INVαF′ (Invitrogen, Carlsbad, Calif.) was used for plasmid preparations. Luria-Bertani medium (Miller 1972) with ampicillin was used for selection for transformed E. coli cells.

Fermentation conditions. In the fermentation experiments, the synthetic grape juice medium “Minimal Must Media” (MMM) (Spiropoulos et al. 2000) was used with 208 mg of nitrogen equivalents/liter. The nitrogen level was generated using 0.2 g of L-arginine/liter and 0.5 g of ammonium phosphate/liter. Fermentations were initiated at a density of 1.33×10⁵ cells/ml by inoculation with stationary-phase cells from a culture pre-grown in MAIM starter medium. Fermentations were conducted in 500-ml Erlenmeyer flasks containing 300 ml of medium. Each flask was outfitted with a silicone stopper with a lead acetate tube attached. The flasks were incubated at 25° C. with shaking at 120 rpm. Fermentations were monitored for seven days using weight loss as an estimate of CO₂ production.

Results

Characterization of hydrogen sulfide production of the deletion strains. In order to assess the hydrogen sulfide production of the entire set of deletion strains they were initially all plated on BiGGY agar and the color of the colonies evaluated. The colonies were white, light tan, tan (parental strain color), light brown, brown or black. Four colonies were white, 258 were light tan, 4478 were tan, 59 were light brown, 28 were brown and one colony was black. The four deletants yielding white colonies were in MET10, MET8, MET5 or MET1. We also identified HOM2, HOM6 and SER33 as possibly playing a role in the suppression of hydrogen sulfide formation (Linderholm et al. 2006).

Identification of the gene responsible for whiteness in a native strain. UCD932, a native strain isolated from Italy (Mortimer et al. 1994) is a white non-H₂S producer on BiGGY agar. To identify the gene that is responsible for its white phenotype, it was mated with each of the white deletion strains. Only one strain failed to complement the white phenotype of UCD932, YFRO30W BY4742.

When a vector carrying the wild type copy of MET10, pAL51 (MET10^(S288C)), was transformed into UCD932, it resulted in a strain of UCD932 producing tan colonies but did not lead to sulfide formation (Table 7). This suggested that more than one gene, possibly in combination with MET10, is responsible for the low-H₂S production phenotype of UCD932.

TABLE 7 Properties of Hydrogen Sulfide Production Fermentations Transformed with MET10 Maximum Fermentation Total H₂S Strains^(a) Rate (g/h)^(bcd) (μg) UCD932 vector 0.437 <1 UCD932pMET10^(S288C) 0.446 <1 UCD932pMET10^(UCD932) 0.408 <1 ^(a)Vector: pYC130; pMET10^(S288C): pAL51; pMET10^(UCD932): pAL52 ^(b)The maximum fermentation rate was calculated from the fermentation rate data by using time points corresponding to the steepest decline in weight. ^(c)Values represent the average of independent determinations of two replicates. ^(d)Fermentations reached dryness (defined by <0.5% sugar remaining).

Sequence analysis of genes in the sulfate reduction pathway. It was demonstrated previously that UCD932 carries mutations in CYS4 and MET6, both encoding for important enzymes in the sulfate reduction pathway (Linderholm et al. 2006). However, introducing wild type alleles to this background did not alter the low H₂S producing characteristic. It was therefore interesting to identify what other mutations this strain might carry in other genes in the pathway. Several genes from the sulfate reduction pathway, MET10, HOM2, HOM6, SER33, MET1, MET5 and MET8 were sequenced from UCD932 as well as from several other native and industrial strains that vary in color on BiGGY agar and in H₂S production in synthetic juice (Spiropoulos et al. 2000) to assess the genetic diversity of the sulfate reduction pathway (a sequence alignment of MET10 from various Saccharomyces strains is found in FIG. 2). MET10p amino acid differences is shown in Table 8.

TABLE 8 MET10p Amino acid differences Amino acid positions 135 172 314 475 511 590 662 896 Consensus T None P D None None T P Modification^(a) N T P or S A I K K S (Strains) (UCD932) (UCD522, (UCD940) (UCD938, (UCD934, (UCD934, (UCD932) (UCD956) T or N 932, 940, 942) 950, 957) 950, 957) (UCD940) 938, 942, T E 956) (S288C, (S288C, A UCD932, UCD522, (S288C, 938, 939, 932, 938, UCD934, 940, 942, 940, 942, 957, 950) 956) 956) A or T T or I Q (UCD939) (UCD522) (UCD939) ^(a)Strains with two amino acid possibilities indicate that the strain carries two alleles.

Sequence analysis of MET10 (a component of the enzyme sulfite reductase) demonstrated that it is not conserved amongst yeast strains (Table 9). Six alleles, different from that of S288C, were found in the ten strains that were sequenced. They were loosely grouped by color on BiGGY and H₂S production. UCD934, UCD957 and UCD950, tan H₂S producers, carried the identical allele. UCD938 and UCD942, tan non-H₂S producers carried the same allele. UCD522 and UCD940, brown H₂S producers, were heterozygous but both alleles were identical for those found in other strains. UCD932 and UCD956, white non-H₂₅ producers, and UCD939, a tan H₂S producer, each carried different alleles.

TABLE 9 Properties of Hydrogen Sulfide Production Fermentations with different MET10 Maximum Fermentation Rate Strains Allele (g/h)^(abc) Total H₂S (μg) UCD932 MET10^(S288C) 0.37 <1 MET10^(UCD932) 0.34 <1 MET10^(UCD950) 0.41 <1 BY4742 MET10^(UCD950) 0.26 <1 UCD950 MET10^(S288C) 0.42 32 MET10^(UCD932) 0.40 <1 MET10^(UCD950) 0.41 29 UCD939 MET10^(S288C) 0.46 <1 MET10^(UCD932) not viable MET10^(UCD939) 0.35 41 UCD940 MET10^(S288C) 0.40 54 MET10^(UCD932) 0.42 <1 MET10^(UCD940) 0.42 49 ^(a)The maximum fermentation rate was calculated from the fermentation rate data by using time points corresponding to the steepest decline in weight. ^(b)Values represent the average of independent determinations of two replicates. ^(c)Fermentations reached dryness (defined by <0.5% sugar remaining).

The other genes in the sulfate reduction pathway were shown to be more conserved. There were no differences in the amino acid or DNA sequence in HOM2 (encodes for aspartic beta semi-aldehyde dehydrogenase), one amino acid difference in HOME (encodes for homoserine dehydrogenase) in UCD932, one amino acid difference in SER33 (encodes for 3-phosphoglycerate dehydrogenase) between S288C and all of the other wine strains and several amino acid differences in MET1, MET5 and MET8 (all components of the sulfite reductase enzyme)).

Swapping of MET10 alleles. The genetic diversity of MET10 alleles and the apparent correlation with H₂S production and colony color supported the hypothesis that genes in the sulfate reduction pathway may be responsible for H₂S phenotype in wine strains, since color on BiGGY agar is loosely correlated with H₂S production through detection of sulfite reductase activity. The effect of MET10 on H₂S production in H₂S producing strains was therefore evaluated. The MET10 alleles of H₂S producing yeast strains were replaced with the allele MET10^(UCD932) (FIG. 3). The native MET10 genes in UCD950, UCD940, UCD939, UCD522 and UCD932 were deleted with a KanMX or HphMX cassette and then the KanMX or HphMX cassettes were replaced with a MET10 allele from UCD932, S288C or their own alleles as a control. All of the strains carrying the MET10^(UCD932) fermented at the same rate as the parental and control strains but became non-H₂S producers and were lighter in color on BiGGY agar. The strains carrying an allele from either S288C or their own allele maintained their H₂S producing phenotype (Table 9).

UCD939 strains carrying the MET10^(UCD932) allele were not restored to methionine prototrophs, in contrast to the other wine and commercial isolates. This may be explained by the presence of other mutations that this strain carries in the sulfate reduction pathway. UCD939 has two mutations in the genes encoding other subunits in the sulfite reductase enzyme. The addition of a third mutation may lower the activity of the sulfite reductase enzyme drastically so there is decreased sulfide available to be incorporated into sulfur containing amino acids, such as methionine or cysteine. Thus the strain cannot grow on plates without methionine. There may also be effects of the accumulation of toxic intermediates upstream of sulfite reductase because the repression of the sulfate pathway has been relieved by the absence of sulfur containing amino acids such as S-adenosyl methionine. However the strain was viable when the MET10^(S288c) allele was substituted for its own allele, the color of the strain on BiGGY changed from tan to white and its H₂S production was significantly reduced.

UCD522, a commercial wine strain that has been characterized as an aneuploid (Bakalinsky and Snow 1990) an imbalance of chromosome number leading to cell death upon sporuation. Therefore both alleles needed to be individually disrupted (FIG. 3) as opposed to knocking out one allele then sporulating the strain to gain a homologous knockout as was done with the other strains. A MET10 allele was transformed into the knockout strains and then it was sporulated to gain two strains that were G418^(R)/hphNT1^(R) and two strains that carried the MET10 allele. Each strain was used in the experiments to observe if there were any inconsistencies due to the genetic manipulations (Table 10). The strains fermented to completion and behaved as expected in terms of H₂S production. Each of the strains carrying the drug resistant marker were methionine auxotrophs and did not produce H₂S. The strains carrying either the MET10^(S288C) or MET10^(UCD522) allele produced H₂S and the strain carrying the MET10^(UCD932) did not produce H₂S.

TABLE 10 Properties of Hydrogen Sulfide Production Fermentations of Heterologous strain UCD522 Maximum Fermentation Rate Strains^(a) Allele (g/h)^(bcd) Total H₂S (μg) UCD522-1A met10Δ::KanMX4 0.35 <1 UCD522-1B MET10^(S288C) 0.35 16 UCD522-1C MET10^(S288C) 0.42 33 UCD522-1D met10Δ::KanMX4 0.24 <1 UCD522-1A MET10^(UCD522) 0.43 26 UCD522-1B met10Δ::hphNT1 0.24 <1 UCD522-1C met10Δ::hphNT1 0.34 <1 UCD522-1D MET10^(UCD522) 0.38 4 UCD522-1A MET10^(UCD932) 0.36 <1 UCD522-1B MET10^(UCD932) 0.37 <1 UCD522-1C met10Δ::KanMX4 0.36 <1 UCD522-1D met10Δ::KanMX4 0.22 <1 ^(a)A, B, C, D- designate the different spores. ^(b)The maximum fermentation rate was calculated from the fermentation rate data by using time points corresponding to the steepest decline in weight. ^(c)Values represent the average of independent determinations of two replicates. ^(d)Fermentations reached dryness (defined by <0.5% sugar remaining).

We also replaced the KanMX cassette in YFR030W BY4742 and UCD932 with the MET10 allele from UCD950 and both are tan on BiGGY agar but neither are H₂S producers. Crosses between BY4742 or UCD932 and UCD950 indicated that there are at least four to five alleles segregating for H₂S production.

Discussion

One of the possibilities for the observed naturally arising differences in sulfide production in S. cerevsiae is the occurrence of genetic alterations of the expression or activity of enzymes in the sulfate reduction pathway. The sulfate reduction pathway displays complex regulation (Mountain et al. 1991) and an increase in one enzymatic activity may be buffered by changes in the activity of other proteins within the pathway.

Previous research in our lab identified, in a native non-H₂S producer UCD932, several alleles within the sulfate reduction pathway. However, we demonstrated that those particular alleles alone are not responsible for the H₂S phenotype (Linderholm et al. 2006). In our screen of the deletion collection for suppressors of H₂S formation, we identified several other genes in the sulfate reduction pathway in that role, HOM2, HOM6, SER33, MET1, MET5, MET8 and MET10 (Linderholm et al. 2006). When those genes were sequenced in UCD932 and other native and industrial yeast strains that vary in H₂S production, it was revealed that UCD932 carried different alleles in five of the nine genes, including CYS4 and METE (Linderholm et al. 2006). There were many alleles of MET10 found within the collection of strains that was sequenced.

In this study it was demonstrated that MET10 plays an important in the role of H₂S formation, while it alone is not responsible for the non-H₂S formation phenotype in UCD932; it dramatically alters the H₂S phenotype in other H₂S producing strains. In the experiments described above, MET10^(UCD932) was successfully swapped for native alleles in three H₂S producing strains and this changed them into non-H₂S producers. These results have many positive implications for the wine industry because of the ability to construct commercial strains with reduced sulfur production in any genetic background by transferring the appropriate alleles or to predict the H₂S production characteristic for any strain of Saccharomyces cerevisiae. Both techniques would be quite simple and useful for winemakers.

The experiments to swap in MET10^(UCD932) in UCD939 were unsuccessful; UCD939 MET10^(UCD932) was not viable on plates deficient in methionine. However it can be explained by other mutations that it carries in the sulfate reduction pathway. UCD939 carries two mutations in the genes encoding other subunits in the sulfite reductase enzyme and although it was not a methionine auxotroph, the addition of a third mutation may lower the activity drastically so it becomes a methionine auxotroph because enzymes downstream cannot boost their activity enough to compensate. The regulation of sulfate reduction by sulfur containing amino acids may also fall apart because the strain can no longer produce methionine and toxic intermediates can accumulate above the sulfite reductase enzymes and also make the strain unviable.

UCD522 was characterized as an aneuploid strain (Bakalinsky and Snow 1990). When the MET10 gene was sequenced in UCD522 it was observed that UCD522 is a heterozygous strain, it carries two alleles of MET10. There may have some type of association between the components that led to the inability to genetically manipulate it like the other strains. It is possible that some type of complex forms between the MET10 alleles or proteins it encodes that does not allow it to segregrate appropriately during sporulation if only one of the alleles is deleted. However when each allele is replaced individually, the strain can sporulate properly. The UCD522 MET10^(UCD932) was sporulated to give two strains that were G418^(R)/hphNT1^(R) and methionine auxotrophs and two strains that were drug sensitive and carried the MET10 allele. Each strain fermented to completion and their H₂₅ characteristic was as expected.

Example 3 Further Characterization of the MET10p Allele of UCD 932

As demonstrated in Examples above, the MET10 allele present in the yeast strain UCD932 is able to convert a high hydrogen sulfide (H₂S) producing strain into a strain that produces no detectible H₂S. This was clearly shown with the high producing strains UCD522 and UCD950, which produced no detectible H₂S when carrying the MET10 allele from UCD932. The ability to convert a strain to a low H₂S producer has implications in any industry that uses yeast including the wine, brewing, and fuel ethanol industries. In addition to presenting a problem for the final product by adding a strong rotten egg smell, the CO₂ created in the fermentation is often a useful byproduct, either to be sold as the gas itself or to be used as a motor gas for the movement of product (brewing). Therefore preventing the gas from smelling of rotten eggs has clear benefits.

The previous work determined that the MET10 alleles from UCD932 and UCD950 differ by six nucleotides, five of those changes result in changes in the primary protein sequence (see, FIG. 2). To further characterize the UCD932 MET10 allele, the native alleles of MET10 were cloned into the shuttle vector pUG6. The Quick Change PCR mutagenesis technique was used to make single nucleotide changes (see, e.g., Cormack, B. and Castano, I. (2002) Introduction of Point Mutations into Cloned Genes. Methods in Enzymology (350) 199-218). In separate reactions, the technique was used to convert one nucleotide difference into the similar nucleotide of the other allele. For example, UCD932 MET10 has an adenine at position 404 while UCD950 has a cytosine. The change of the UCD950 cytosine at position 1985 for an adenine was found to be necessary and sufficient for the loss of sulfide production in the UCD950 background. (Table 11) The conversion of the adenine in the UCD932 allele to the cytosine of 950 eliminated the ability of the UCD932 protein to eliminate sulfide production (Table 11). Therefore the single change of the threonine at position 662 to a lysine residue results in the creation of a modified Met10 protein leading to reduced sulfide release.

TABLE 11 H₂S Production by Different MET10 Alleles Nucleotide Strain Produces MET10 Allele at 1985 Background H₂S? UCD932 Adenine UCD522 No UCD932 Adenine UCD932 No UCD932 Adenine UCD940 No UCD932 Adenine UCD950 No UCD950 Cytosine UCD522 Yes UCD950 Cytosine UCD932 No* UCD950 Cytosine UCD940 Yes UCD950 Cytosine UCD950 Yes UCD932 1985 A-C Cytosine UCD522 Yes UCD932 1985 A-C Cytosine UCD932 No* UCD932 1985 A-C Cytosine UCD940 Yes UCD932 1985 A-C Cytosine UCD950 Yes UCD950 1985 C-A Adenine UCD522 No UCD950 1985 C-A Adenine UCD932 No UCD950 1985 C-A Adenine UCD940 No UCD950 1985 C-A Adenine UCD950 No *The 932 strain background has other determinants of H₂S production and does not produce H₂S under any conditions testes.

Example 4 Allelic Differences at Position 1985 of the MET10 Gene Determine Hydrogen Sulfide Production by Saccharomyces cerevisiae

The MET10 allele of strain UCD932 leads to the inability to produce hydrogen sulfide (H₂S) when used in an allele replacement strategy to replace the native allele in commercial and native isolates of wine yeast. This allele was found to contain several base pair changes leading to differences in amino acid sequence of the encoded protein. These amino acid changes have been evaluated to determine which one(s) impact the ability to produce H₂S.

To identify the exact mutation or combination of mutations responsible for these dramatic differences in H₂S production, we cloned the MET10 alleles from UCD932 and UCD950 and systematically converted each single base difference to the base of the opposite allele using Site Directed Mutagenesis. The resulting alleles were identical to the parent allele with the exception of the single swapped base change. The modified alleles were then inserted back into both strains and BiGGY agar was used as an indicator of a change in sulfite reduction and likely H₂S production. A single base change at position 1985 was identified by this screen as the mutation responsible for the change in colony color. These strains were examined for H₂S production in duplicate during small scale fermentations in synthetic wine juice. 10 mL of synthetic wine media was inoculated with the respective strains and H₂S was detected by the use of lead acetate columns after four days of fermentation. The unchanged UCD950 MET10 allele and the UCD932 allele with the mutation to the UCD950 allele at position 1985 (932 MET10 1985 A-C) resulted in H₂₅ production while the unchanged allele UCD932 MET10 and the UCD950 allele with the change to UCD932 at position 1985 (950 MET10 1985C-A) resulted in no detectable H₂₅ production. These results indicate that the single base change at position 1985 is a key determinant of the difference in H₂S production of these alleles. These results were then strengthened by examining the production of H₂S when the single mutant alleles were place into two high H₂S producing commercial strains UCD522 and UCD940. Both of these strains produced H₂S with the 932 MET10 1985A-C allele but no H₂S was detected with the 950 MET10 1985C-A allele. The results are summarized in Table 11, above.

This study demonstrates that a single base pair change at position 1985 in the MET10 allele dictates the production of hydrogen sulfide. The nucleotide difference at 1985 changes the encoded amino acid, thus any change in the surrounding nucleotide sequence that changes the encoded amino acid will likely also eliminate H₂S production. The threonine present in the high producing alleles may act as regulatory point that changes the flux of the pathway as amino acid residues containing a phosphate group can be regulated via phosphorylation. Amino acid residue 662 is predicted to be within the sulfite reductase catalytic domain (FIG. 6). Analysis of the putative structure of the protein with this change (FIG. 7) indicates that the overall structure of the protein is unaltered, but the local area of the active site surrounding this residue change is affected. Thus this change modifies the protein structure only slightly.

REFERENCES

-   Acree, T. E., E. P. Sonoff, and D. F. Splittstoesser. 1972. Effect     of yeast strain and type of sulfur compound on hydrogen sulfide     production. Am. J. Enol. Vitic. 23:6-9. -   Amoore, J. E. and E. Hautala. 1983. Odor as an aid to chemical     safety: Odor thresholds compared with threshold limit values and     volatilities for 214 chemicals in air and water dilution. J. Appl.     Toxicol. 3:272-290. -   Bakalinsky, A. T. and R. Snow. 1990. The chromosomal constitution of     wine strains of Saccharomyces cerevisiae. Yeast. 6:367-382. -   Baudin, A., O. Ozier-Kalogeropoulos, A. Denouel, F. Lacroute, and C.     Cullin. 1993. A simple and efficient method for direct gene deletion     in Saccharomyces cerevisiae. Nucleic Acids Res. 21(14): 3329-3330. -   Bohlscheid, J. C. and C. G. Edwards. 2004. Interactive effects of     nitrogen and biotin on yeast growth, fermentation rate, and volatile     production. Am. J. Enol. Vitic. 55:310 A. -   Eschenbruch, R., P. Bonish and B. M. Fisher. 1978. The production of     H₂S by pure culture wine yeasts. Vitis 17:67-74. -   Giudici, P. and R. E. Kunkee. 1994. The effect of nitrogen     deficiency and sulfur-containing amino acids on the reduction of     sulfate to hydrogen sulfide by wine yeasts. Am. J. Enol. Vitic.     45(1): p. 107-112. -   Guthrie C. and G. R. Fink. 1991. Methods in Enzymology 194:3-21. -   Hansen J., T. Felding, P. F. Johannesen, J Piskur, C. L. Christensen     and K. Olesen. 2003. Further development of the cassette-based pYC     plasmid system by incorporation of the dominant hph, nat and AUR1-C     gene markers and the lacZ reporter system. FEMS Yeast Research.     4:323-327. -   Hoffman, C. S., and F. Winston. 1987. A Ten-minute DNA preparation     from yeast efficiently releases autonomous plasmids for     transformation of Escherichia coli. Gene (Amsterdam). 57:267-272. -   Inoue, H., H. Nojima, and H. Okayama. 1990. High efficiency     transformation of Escherichia coli with plasmids. Gene 96:23-28. -   Jiranek, V., P. Langridge and P. A. Henschke. 1995. Regulation of     hydrogen sulfide liberation in wine-producing Saccharomyces     cerevisiae strains by assimilable nitrogen. Appl. Environ.     Microbiol. 61:461-467.

Linderholm, A. L., T. L. Olineka, Y. Hong and L. F. Bisson. 2006. Allele diversity among genes of the sulfate reduction pathway in wine strains of Saccharomyces cerevisiae. Am. J. Enol. Vitic. 57(4):431-440.

-   Miller, J. H. 1972. Experiments in molecular genetics, p. 431-435.     Cold Spring Harbor Laboratory Press, Plainview, N.Y. -   Mortimer R. K., P Romano, G. Suzzi and M. Posinelli. 1994. Genome     renewal: A new phenomenon revealed from a genetic study of 43     strains of Saccharomyces cerevisiae derived from natural     fermentation of grape musts. Yeast. 10(12): p. 1543-1552. -   Mountain, H. A., A. Bystrom, J. T. Larsen and C. Korch. 1991. Four     major transcriptional responses in the methionine/threonine     biosynthetic pathway of Saccharomyces cerevisiae. Yeast 7:781-803. -   Nickerson, W. J. 1953. Reduction of inorganic substances by     yeast. 1. Extracellular reduction of sulfite by species of Candida.     The Journal of Infectious Diseases. 93: 43-48. -   Olesen K., P. Franke Johannesen, L. Hoffmann, S. Bech Sørensen, C.     Gjermansen and J. Hansen. 2000. The pYC plasmids, a series of     cassette-based yeast plasmid vectors providing means of     counter-selection. Yeast 16(11):1035-43. -   Rauhut, D. and H. Kurbel. 1994. The production of H₂S from elemental     sulfur residues during fermentation and its influence on the     formation of sulfur metabolites causing off-flavors in wines.     Wein-Wissenschaft. 49:27-36. -   Schiestl, R. H., and R. D. Gietz 1989. High efficiency     transformation of intact cells using single stranded nucleic acids     as a carrier. Curr. Genet. 16:339-346. -   Sherman, F., G. R. Fink, and J. B. Hinks. 1986. Methods in yeast     genetics. Cold Spring Harbor Laboratory, Cold Springs New York. -   Sherman, F., G. R. Fink and C. W. Lawrence. 1974. Methods in yeast     genetics: a laboratory manual. Cold Spring Harbor Laboratory, Cold     Spring Harbor, N.Y. -   Spiropoulos, A., J. Tanaka and L. F. Bisson. 2000. Characterization     of hydrogen sulfide formation in commercial and natural wine     isolates of Saccharomyces. Am. J. Enol. Vitic. 51(3): 233-248.

Spiropoulos, A. and L. F. Bisson. 2000. MET17 and hydrogen sulfide formation in Saccharomyces cerevisiae. Appl. Environ. Microbiol. 66(10):4421-4426.

-   Stratford, M. and A. H. Rose. 1985. Hydrogen sulfide production from     sulfite by Saccharomyces cerevisiae. J. Gen. Microbiol.     131:1417-1424. -   Thoukis, G. and L. A. Stern. 1962. A review and some studies of the     effect of sulfur on the formation of off-odors in wine. Am. J. Enol.     Vitic. 13(3):133-140. -   Tokuyama, T., H. Kuraishi, K. Aida and T. Uemura. 1973. Hydrogen     sulfide evolution due to panthothenic acid deficiency in the yeast     requiring this vitamin, with special reference to the effect of     adenosine triphosphate on yeast cysteine desulfhydrase. J. Gen.     Appl. Microbiol. 19:439-466. -   Wainwright, T. 1970. Hydrogen sulphide production by yeast under     conditions of methionine, pantothenate or vitamin B6 deficiency. J.     Gen. Microbiol. 61:107-119. -   Walker, M. D. and W. J. Simpson. 1993. Production of volatile     sulphur compounds by ale and lager brewing strains of Saccharomyces     cerevisiae. Letts. Appl. Microbiol. 16:40-4.

It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, accession numbers, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes. 

1. A method for reducing H₂S levels in a fermentation medium, the method comprising contacting the fermentation medium with a yeast cell comprising a polynucleotide encoding a MET10 polypeptide that does not catalyze the conversion of sulfite into sulfide, wherein the amino acid at position 662 of the MET10 polypeptide is not threonine.
 2. The method of claim 1, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is not threonine.
 3. The method of claim 1, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is lysine.
 4. The method of claim 1, wherein the polynucleotide shares at least 95% sequence identity with a nucleic acid sequence of SEQ ID NO:1.
 5. The method of claim 1, wherein a fermentation product having no detectable levels of H₂S is produced, wherein the fermentation product is selected from the group consisting of: wine, beer and champagne.
 6. The method of claim 1, wherein the yeast cell is a Saccharomyces cerevisiae cell.
 7. The method of claim 6, wherein the yeast cell is a wine yeast strain selected from the group consisting of: Prise de Mousse, Premier Cuveé, French Red, Montachet, Lallemand K1, Bordeaux, UCD522, UCD940, Ba25, Ba126, Ba137, Ba220, Bb23, Bb25, Ba30, Bb32, Bb19 and Bb22.
 8. The method of claim 1, wherein the fermentation medium is selected from the group consisting of: a must and a wort.
 9. The method of claim 1, wherein the fermentation medium is a grape juice must.
 10. An expression vector comprising a polynucleotide encoding a MET10 polypeptide that does not catalyze the conversion of sulfite into sulfide, wherein the amino acid at position 662 of the MET10 polypeptide is not threonine, and wherein the polynucleotide is operably linked to an expression control sequence.
 11. The expression vector of claim 10, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is not threonine.
 12. The expression vector of claim 10, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is lysine.
 13. The expression vector of claim 10, wherein the polynucleotide shares at least 95% sequence identity with a nucleic acid sequence of SEQ ID NO:1.
 14. A host cell comprising an expression vector of claim
 10. 15. The host cell of claim 14, wherein the cell is a yeast cell.
 16. The host cell of claim 14, wherein the cell is a Saccharomyces cerevisiae cell.
 17. An improved yeast cell that does not produce hydrogen sulfide comprising an exogenous polynucleotide encoding a MET10 polypeptide that does not catalyze the conversion of sulfite to sulfide, wherein the amino acid at position 662 of the MET10 polypeptide is not threonine, wherein a parent cell of the improved yeast cell produces hydrogen sulfide.
 18. The yeast cell of claim 17, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is not threonine.
 19. The yeast cell of claim 17, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is lysine.
 20. The yeast cell of claim 17, wherein the polynucleotide shares at least 95% sequence identity with a nucleic acid sequence of SEQ ID NO:1.
 21. The yeast cell of claim 17, wherein the yeast cell is a Saccharomyces cerevisiae cell.
 22. The yeast cell of claim 21, wherein the yeast cell is a wine yeast strain selected from the group consisting of: Prise de Mousse, Premier Cuveé, French Red, Montachet, Lallemand K1, Bordeaux, UCD522, UCD940, Ba25, Ba126, Ba137, Ba220, Bb23, Bb25, Ba30, Bb32, Bb19 and Bb22.
 23. An improved yeast cell culture that produces reduced levels of hydrogen sulfide comprising a population of yeast cells, the yeast cells comprising an exogenous polynucleotide encoding a MET10 polypeptide that does not catalyze the conversion of sulfite to sulfide, wherein the amino acid at position 662 of the MET10 polypeptide is not threonine, wherein the improved yeast cell culture produces reduced hydrogen sulfide in comparison to a culture of parent cells.
 24. The yeast cell culture of claim 23, wherein the culture does not produce detectable levels of hydrogen sulfide.
 25. The yeast cell culture of claim 23, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is not threonine.
 26. The yeast cell culture of claim 23, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is lysine.
 27. The yeast cell culture of claim 23, wherein the polynucleotide shares at least 95% sequence identity with a nucleic acid sequence of SEQ ID NO:1.
 28. The yeast cell culture of claim 23, wherein the population of yeast cells comprise Saccharomyces cerevisiae cells.
 29. The yeast cell culture of claim 23, wherein the population of yeast cells are from a wine yeast strain selected from the group consisting of: Prise de Mousse, Premier Cuveé, French Red, Montachet, Lallemand K1, Bordeaux, UCD522, UCD940, Ba25, Ba126, Ba137, Ba220, Bb23, Bb25, Ba30, Bb32, Bb19 and Bb22.
 30. A method of producing an improved yeast cell that produces reduced levels of hydrogen sulfide, the method comprising replacing an endogenous polynucleotide encoding a sulfide active MET10 polypeptide with a polynucleotide encoding a sulfide inactive MET10 polypeptide by introducing into a parent of the yeast cell a polynucleotide encoding a sulfide inactive MET10 polypeptide that does not catalyze the conversion of sulfite to sulfide, wherein the amino acid at position 662 of the sulfide inactive MET10 polypeptide is not threonine, wherein the parent of the improved yeast cell produces hydrogen sulfide.
 31. The method of claim 30, wherein the polynucleotide encoding the sulfide inactive MET10 polypeptide is introduced recombinantly.
 32. The method of claim 30, wherein the polynucleotide encoding the sulfide inactive MET10 polypeptide is introduced by back-crossing.
 33. The method of claim 30, wherein the improved yeast cell does not produce detectable levels of hydrogen sulfide.
 34. The method of claim 30, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is not threonine.
 35. The method of claim 30, wherein the polynucleotide encodes a MET10 polypeptide of SEQ ID NO:3, wherein X at position 662 is lysine.
 36. The method of claim 30, wherein the polynucleotide comprises SEQ ID NO:1.
 37. An isolated polynucleotide capable of distinguishing between the sequences provided in SEQ ID NO:1 or a complement thereof and a nucleic acid encoding a wild type MET10.
 38. An expression vector comprising a polynucleotide of claim 37 operably linked to an expression control sequence.
 39. A host cell comprising an expression vector of claim
 38. 40. The host cell of claim 39, wherein the cell is a Saccharomyces cerevisiae cell. 